What size breaker and wire do I need for a subpanel in my shed?
I've said it before, and I'll say it again. It's tough to beat a 60 ampere sub-panel. You might be able to get by with a 40 amp setup, but it's always nice to have future expansion possibilities. 

At 90 ft., you're probably okay to use 6 AWG copper, or 4 AWG aluminum conductors. You'll have to pull four conductors, either through conduit or via a cable.

Since it's a separate structure, you'll need a grounding electrode system. Which you'll have to bond the grounding conductor from the feeder, and the grounding conductors within the building to. You'll keep the neutral conductors isolated from the grounding conductors, which will likely mean removing a bonding jumper within the panel.