Depends what you want really both sizes have good options.
Firemouths get to about 6 inches but getting upto 7 or 8 are not unusual.
For a 2 foot tank you could look at things like on the basis of it being around 20-25 gallons
Apistogramma (though a trio of these would work better)
Laetacara
Cherckerboards
Nanochromis Transvestitus
Blue Rams (would need a special set up but can do it the tank is based around them) - Any of the variants would work, Electric Blues, Golds etc
Bolivian Rams
You could also get a small school with these of something like Neon Tetras
In a 3 foot assuming its 30-40 gallons a pair of
Rainbow Cichlids
Convicts
Sajica
Honduran Red Point
maybe Firemouths
maybe Elliotii (these two would not be my first choice)
And then a school of a larger tetra or barb like Red Eye Tetras. A trio of Swordtails would work here as well.
Wills
