I bought a 10 gallon nanotank for my girlfriend this Christmas and she loves it. It really depends on what you want to keep in the tank and how much money you want to spend on equipment. You have have a really nice 24 gallon tank with live rock and fish.
You could even have other critters that are not highly dependant on light if you don't want to spend a lot on lighting. Feather dusters, crabs, shrimp, stars, sponges, gorgonians, etc.