Ideal Game Table Size

Lets say you were thinking of building a gaming table, and the room in which it would be set was pretty large. Let's also say that said gaming table would be used mostly for 28mm gaming, but also some smaller scales on occasion. Let's also say it would be used for a variety of games, and by different people, playing anything from ancient mass battles to 20th century urban warfare.

What reasonable size would you aim for?

All of my tables have been 4'x8' but given the chance, I'd like to try something a bit larger in the future. I think 6'x12' would probably be the extreme limit. Maybe 5'x10'?