subwoofer question......what size sub ?

FWIW I have used  Entec, Vandersteen, Velodyne and REL subs with 4 different series of Proac speakers.
My ears like the REL  products. They are fast and accurate, not to be confused with subs that make Top Gun(and Tom) sound great!
They have a very simple attachment method that takes the output from your amps speaker terminals. Very adjustable and I have yet to find a room that they can't be tuned to work in... YMMV.

Get a 10" woofer model and start the learning curve about room resonances and standing waves.
The Proac/REL combo can be very rewarding.
Good Hunting!

m-db...

I first used a Velodyne 12" servo controlled sub circa early 90's.( don't remember the Model)
got talked into it by large mid-fi retailer in Ft Laud. even though the rest of the sys. was krell amp, Vandersteen speakers, Cat pre and Basis Ovation... 
Then I opted for the Vandersteen 2w, one behind each speaker. WAS great BUT... between the Stereo being in the living room and the Krell (class a space heater)  starting to delaminate the teak wall unit I had stuffed it in I was given a spare bedroom for a dedicated listening room. 
That was when the Vandersteens went away, the Proac's came in Response 2 and an amazing set of 36" tall entecs with 3 8" drivers in each side.   Start the learning curve about room size, standing waves and room treatment. Amp's were now 200 watt MFA monos with 6550s . Glorious!!! But had to install second A/C vent and extra ceiling fan over the amp end of the room. Lived in So Fla at the time.
Next up was a pair of the original Electra Amator speakers pre Sumiko days. They were not being imported at the time but I took a leap of faith and was well rewarded!
Next up was a pair of Response 2S's. Did not like running the entec external crossover so settled on a midline/midsize Rel (single) (because of their high level input from the speaker posts on the amp, wanted to avoid the crossover) and was amazed. the MFA's left and were replaced by sonic frontiers Amp 2, and SF line 3 with the SF all tube phono. Amazing....
Then i jumped for a Proac 3.8 pair. A set of Cary 300b signature amps( with WE 300b tubes... wish i still had the tubes) did the hard work and I opted for a pair of RELs, one behind each 3.8. This was a great setup. I had by then invested in room tunes corner cushions and 4 of their tall panels. 

currently using an older B3 Rel with either Proac D2s or Ref 3a De Cappo's . Amps are either SS (First Watt) or yet to be determined stereo tube amp.... something special about Proacs and hi quality tube amps. Very small room so I'm not thinking two RELs just yet.....
After all this hobby can be a sickness....

Wally