Do they have "Mongo Sized" where you shop?
I usually prefer 8", rather than 6". But it also depends on the monitors. If the monitors suck, then listening at low volumes can sometimes be more effective.
Sometimes tracking and mixing with subs is okay, but you have to have some reliable "b" monitor for contant cross reference.
Of course, there's always a chance that the 8's will be a bit boomy in the smaller space, but if you've got sufficient damping and traps, you might be cool.
Just my $ .02