ProAc Response D38 Owners:

Generally, ProAc's sound best well out from both side walls and the back wall with significant toe-in, even to the point of slightly crossing them. For specific distances from the side walls and back wall, divide the room into 5th's, and 7th's and place them in some combination of odd dimensions from each wall. Otherwise do it by ear and put them in the spots that make them seem to disappear the best.

I'd set up the speakers on the long wall. Given the size of that speaker, I'd say you'd want a minimum of 15 feet for the long wall. More would be better. Then try to make sure that none of the dimensions are multiples of each other, i.e. don't make a room that's 16x24 with an 8 foot ceiling.

The good news is that ProAc's sound really bloody good even in the wrong spot in the wrong room, but optimizing things brings much better performance.

I'm running Response 5's in a room that's about 17x20 with 9 foot ceilings and it works very well.

A couple more tips:

Try to have no furniture between the speakers and try to use absorptive material on the wall behind the speakers centered around the height that your eyes look at when seated in your listening chair. Also treat the first reflection points on the side walls.

I'm thinking asking the ProAc Response people about this upcoming event ought ot be very enlightening... I'd start there myself.

As much as is the talk hereabouts on the room vs speaker integration, I'm not terribly certain I'd build a room for one sort of loud speaker only.

I'd be far more tempted to errect such a thing for space instead. A room for one? Two? or Six people regularly? That is a consideration too.... as is upgrading speakers later on.
Neither am I too sure there is some 'cast into stone' parameters for exact room size per pr of speakers... or vice versa. I'd suspect there is some latitude for every speaker set with regard to the room it is capable of adapting to.

Volume is the important thing, not just square footage.... then too is the perspective you want while listening to them. Nearfield? Well back?

There's some immense amount of info on what is the perfect room size as it relates to nodes, peaks and dips, from a purely frequency range perspective.

I'd think you'll find only a most general notion as to what is the optimum sized room for most any speaker... but hopefully you'll get the right info as to your needs.