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I think the 3'x4' is a good size. You can get to the action a little quicker saving a turn or so maneuvering and as Faversham says it's easier to get a lot of terrain on the table for skirmish games.I can't imagine how many buildings I would need for an 8'x6' gangster game.
I only use a 3×4 and a 3×3. Not simply because of space.But it occurred to me whilst watching games at my local club .That most games being played deployed 30 cm in from the board edge . So even just a 4×6 had nearly twelve square feet of unused board from the first dice role.Then if you take into account very few games and gamers play right upto the sides of the board .So effectively other than looking pretty half a 4×6 table isn't actually used for anything other than elbows and coffee cups(so to speak.)So I reduced both the storage and the effort required by building smaller surfaces.
What is odd though is our instincts for rectangular and square gaming surfaces. Try playing on around or oval table top and not layout a four sided board.It can be rather unnerving