my female is currently about 3.5", but i've seen adults the size of a man's hand before.
unless you just particularly want to breed convicts, i'd suggest getting just one. they're pretty active fish in general and you're much less likely to become overrun with babies.
if you are dead-set on breeding cichlids in your 29, then might i suggest looking into other species before getting convicts? jewel cichlids share many of the same personality traits as convicts but are easier to distribute to LFS due to their color. other somewhat common species that might be good to breed would be Bolivian Rams, Curviceps, or Rainbow cichlids.
as someone who started out breeding very common cichlids, it's not worth the trouble unless your LFS sells tons of fish. common cichlids breed fast and easy; you'll quickly be overrun. and it's also hard to sell common stuff over Aquabid because (1) it's mostly specialists shopping there and (2) common stuff is readily available everywhere.
just some stuff to think about.