I can't remember which is the smallest monitor, but a 2x2 would be ok for them while they were young, and as they grow up move them into something abit larger..
my ackies are in a custom made coffee table, 3.5 x 2 and they are happy as at the moment, but in the next 6-9 months i'll be moving them into something bigger that will eventually house 4-6 adult ackies.