>>273773025
That's a very sensible question but the answer is going to change from person to person and depends on the title really. 

Personally I only have about 1.5x1.5 area (And it isn't even square) to play in but I still manage just fine but I know for a fact there are people who absolutely wouldn't even put their HMD on if they couldn't guarantee 2.5x2.5 at least.