Standard size for a decent 2-3k battle is 6x4 foot. My local GW's use 4x4, and of course two can be put together. I would aim for 6x4 if I were you.
Yea, 6x4 is pretty much standard. If you have a modular board, then you can configure it either way, of course.
Gotta be careful with modular boards though. They give a big advantage to armies with guess range weaponry if you don't make the seams invisible.