In other words, that's the wrong approach unless you're an acoustical engineer with a large budget and the right software. Remember it's not just room size to consider, it's furniture, materials, wall treatments, etc.
I would say that 99.9% of people with home theatres, even dedicated ones, work with what they have. If they're good at it, they tweak it as they go along, moving speakers until it sounds good.
That said, with the right speakers and only a bit of tweaking, it still sounds pretty damn good, eh? Call that 80% (I've been watching a lot of Star Trek, where they quantify unquantifiable things with percentages). Hell, even in my living room, which is an actual living room with people and things moving around in it all the time and extremely non-optimal placement of the center, it sounds pretty damn good (65.2%, Captain!)
I am the Doctor, and THIS... is my SPOON!