You didn't quite mention what gear you were considering using in your room..this has a lot to do with what's workable/not workable in a given room..especially smaller rooms like the one your considering at 12.5X18! ]
My vast experience with dealing with all sorts of rooms sized, shapes, etc, is that, yes, your larger rooms(in general) tend to be easier to work with from a frequency response and balance stand point, and gives you more options for different type of gear/seating/speaker placement scenarios for best performance! Small rooms tend to be even more difficult to set up any given speaker/seating arrangement, to yield flat response from the speakers, especially if you need more than two seating/listening possitions. And make no mistake about it, you will have a tough challenge getting good balanced speaker response in that acoustically relatively small environment of 12.5X18X10!!! I've done enough high end custom theater/listening rooms of that size range to know better. If you end up wanting to push your couch or chairs towards the back of the room,so you can get further away from a big screen for example, that makes things even more challenging sonically!
I would suggest, that you consider either a one chair or two chair set up in a room of that size, as the primary listening spots, and try to balance things around that(those) chairs! Trying to get more chairs in there will be difficult to get great sound from all of them(although you can have some additional seating possibly with some sacrifice in the sound ultimately). If your mostly a "solo" watcher/listener, I think, with a little knowledge, experience, OR KNOWLEDGABLE HELP/ASSISTANCE, you could possibly get reasonably flat frequency response from all the speakers in a room of 12.5X18X10, yes. Then treating room reflections, and balancing out reverb and overall all balance in the room will take some more effort.
You didn't mention if the room was a "closed in" room, or "open to other rooms and openings" type room. This makes a great deal of difference acoustically!..so be aware of this. Infact, as a designer and acoustical engineer myself, I'd want to see that actual layout of the room first!
Your first priority in a room of that size, if music(I suppose 2 channel music mostly, yes?) is your priority especially, should be, one,getting ALL THE SPEAKERS BALANCED for even and equal frequency response(mainly bass in considered here!), and integrating the woofer(s) well also!!!! If you can't at least get your main 2 music speakers for your listening set up well, then you can forget really fantastic sound anyway!..it's just not going to happen. I would recommend setting up ONE SPEAKER AT A TIME(in mono/summed stereo)and get each one right from the main listening possition(s)! Start with left and right(this will take some learning/trial and error), then go to sub!(yes, sub!...so you can get the sub to integrated properly with the left and right and your seating possition!). Then, once you have that combo going perfectly, you can play around with the center channel to balance with the left/right/sub combo!. Then, do rears to balance with the sub possition, and for flat frequency response as well!
If you can pull that balancing act off, you'll be doing EXTERMELY WELL SONICALLY!..that supposing you have nice sounding gear in the first place(gear 40-50%/room and setup 40-50%/tweeks/5-10% roughly). Most people, again, CANNOT GET THE SET UP DONE WELL!...so the results suffer badly! Trust me, I've been around 1000's of setups over the years...thousands!
Placing speaker in a room, getting the seating possitions right, and playing with acoustics, is all science, skill, patience, and experience! learn how to do this well, and you can virtually work with any system and room!...within reason of course.
Now in regards to that 12.5 X 18 X 10 foot room...I see one potential problem, in that you may experience(if you do no mod's to the dimmensions)some frequency response build up at certain bass modes, causing a boomy sound at certain frequencies. This usually makes speaker and setting placement options difficult. It's difficult to see exactly all of what's going on hower, since I'm not seeing the actual room shape. Also, STAY AWAY FROM THE BACK WALL WITH YOUR CHAIR(S) ALSO! This will make getting anything except boomy peaky sound all but extremely difficult to get rid of! Try putting your chair in listening spots of either 1/5's or 1/3's from back to front, or left to right! If it's mostly a "solo/one-chair" listening room, just for your, then you can consider placing your chair in the middle of the left/right wall, and place the chair in either 1/3rd length possition, or 1/5th lenght possition from back to front. Then from there, you can play with speakers balancing from that seating possition! If you do two seats, then keep the two seats symetrical, and they will both sound the same, as long as room is symetrical).
After you can get the speaker system in good foundational positions from your seating spots, you can concentrate on treating reflections and echoes, reverb, whatever, etc.
Another thing to consider aslo is your type of viewing monitor/projector! If you use a big screen RPTV, you must consider that you will not be able to sit so close to it!(although you can sit closer than you used to with newer sets and higher def material) This often forces you to sit back further in the room. and the bigger the screen the more it will put you back towards the back wall, causing acoustic challenges, So be careful.
Even after proper monitor calibration, you still can't get as close to the screen, and as big of a picture with a rear projection big screen as you can with an overheard/front projector! If you go the way of a front projector(STRONGLY RECOMMENDED), you can get a much bigger, and better overall picture, wich you can sit closer to!!!...giving a much better home theater experience really. You can also get teh screen flush on the front wall, makeing for even more room for speakers, gear and seats!...another thought.
Incase you weren't awarer, rhe reason, even witih the same resolution sources, that you must sit further away from a big screen than good front projector set-up, is that the lenticular screen on a RPTV is too vissable up close, and you see the grid too easliy. In order to not be distracted by this, it forces you to sit back further..this is not a problem with overhead/front projectors!. In addition, with the front projector,the wow factor is just much greater with the larger picture, even though you mentioned music as a priority.(you'd be blown away with a good projetor and screen combo!!!!...it's really good potentiall...it's a thought)
For that room your sonsidering, for example, I would not recommend anything above 50" 16X9(max) HD for considering in RPTV bigscreen! You'll otherwise be forced against the back wall if you want a great picture.
Anyway, my suggestion, if you really want to do this right, and not make it an on going hobby, is to pay someone who's experienced to either do it for you, or at least help you!...whether it be via email/fax/phone, or in person!!!
There's simply SO SO SOOOOO much to consider in making systems sound an look great, in any given set-up, with any given room, with any given gear! The list is long in considerations to adress, indeed! If it's simply something you want to tackle your self, you will do well to learn learn learn, experiement, experiment, etxperiment, and do lots of reading! Picking up back issues of Stereophile GHT magazine, and reading all of Russ Herschelmanns "home Theater Architect" couldn't hurt either. I also strongly recommend F. Alton Everests Master Hand book of Acoustics as well.
Still, there's no replacment for experience. If you just wanted it right from the beggining, so you could relax and just enjoy your system, I highly suggest seaking professional assistance.
Anyway, it's all good...it's just that, with experience, knowledge, and some skill, it's WAY better! When you consider all the variables, and speaker/gear choices, and settings/calibrations, and proper set up condiderations, acoustics,and a virutally endless mirriad of other considerations of importance, etc., getting world class results is a daunting task for the average "newbbie" to the game!...just the way it is. Heck, even seasoned audiophiles often don't do it so well. Infact, that's the norm mostly!!