2.5gal - 10gal...its probably going to cost you about the same, give or take a few bucks.
I have a 2.5gal with no fish just coral, i love it. Very easy to maintain yet still a very interesting colorful tank. However, if you went this route you would have to upgrade your lights and buy the coral which will up the cost a little.
I've seen 1gallon picos before, but from experience with my 2.5...i wouldn't go smaller. Its such a tight space and i'm always wanting more. Hard to work inside of the tank. I can't even imagine a >1gal tank.
If you go below something like a 10gal or maybe 5gal your best bet is a goby like listed above.