You always have to buy for room size... it's just that sometimes you don't know how big the room's gonna be so you have to guess a little
Seriously, I understand the new house probably isn't picked out yet but are you thinking about having the system in a relatively small room (like a den etc...) or a relatively large room (1/2 the basement etc...) ?
One good indicator is the size of TV you are thinking about, since that normally affects how far away you will be from the speakers, which in turn helps to determine the best speaker size etc...
