green sunfish are usually very aggressive. i would not put anything in that tank that you valued highly,
for care think Oscar, the can get max about 12" or so, i would think that 10" - 9" would be more common.
a 55 gal for one, possibly a pair if you are willing to do frequent and heavy w/c's
temp should be under 78 f, probably around 68-72 is ideal, they can stand low 80's for weeks (if you need to do an ick treatment, or parasite purge)
feed them floating pellets for a staple, mix in crickets, worms, fd/frozen shrimp, beef heart once a week or so to supplement diet. I have gone to feeding my warmouth and shadow bass every other day (once they both got about 3" or so)
these fish like to build "beds" (they will point their heads at the top of the tank and "fan" with their tail in a swishing motion, until they clear a space about 8"-12" in diameter, if you have plants leave them a space to build a nest, or they will rip up foliage to make themselves a spot; i would think that otherwise they would leave most plants alone.
I would recommend pea gravel for a substrate, sand can get stirred up and end up in the impeller of many filters
they are great fish, tons of personality, they will quickly loose all fear of you, and come to the glass and beg, almost dog like