Does anyone know if there's an ideal sized tank for a planted cherry shrimp setup? I know cherry shrimp can be kept in somewhat small tanks but how small is too small you think?

I would probably say 5 gallons at a minimum. Firstly because the shrimp need enough space to feel comfortable to explore and breed - small though they are they still need a bit of space. And second because 5 gallon tanks are 'big' enough to be sort of easily managed. I'm guessing a 2 or 3 gallon will need more water changes, filtration might be an issue in such a small tank (don't want the flow to be too much for the shrimp), and so on. But maybe other people with tiny tanks can chime in.