One of the easiest (and most overlooked ways) to determine the best room size is to identify the equipment and furnishings that will be in the room. No need to build a large room if you don't need the space, or a small room if you have a lot of stuff to put in it! As for the "best room size" question, there really isn't any super calculator available that will provide this information. One thing that I've identified over the years spent working with both audio and home theater set ups is that it seems to be easier to properly voice a multi-channel system if the room is approx. twice as long as it is wide and the walls are perpendicular.
Also, just as important as the size of the room is to make sure you take the time to set the room up right! This means constructing the room using quality sound dampening materials both in the walls / ceiling and on the walls / ceiling. Theater rooms sound their best when the walls and ceiling are sealed tight. If you really plan on using 6 subs in your system, serious acoustical treatments are a must! Not just to keep the sound away from your neighbors (lol), but to keep all that bass from sounding muddy as well as preventing unwanted room reflections.