i like big tanks. i think 75gal +/- is nice for a established community but i think a 40 or 50gal would be nice aswell. i think a 120+ would be HUGE! when i think of community tanks i think of neons, rummynose etc, small fish and just think a huge tank with these in it is overkill. if anyone has a 90gal+ community tank id love to see pics, maybe id change my mind.
I agree with Nadine, get a big tank to start, i bought a 40gal to start and within a few months i bought a 120, then a 72 bowfront a few months after, then a 40gal for breeding. make sure u dont cheep out on the filter too! just remember to find out if your floor (if its not a basement concrete floor) can hold the weight, i had to reinforce mine for a 120gal.