What size breaker and wire do I need for central air conditioning?
It's going to completely depend on the unit that's being installed, but commonly it'll be 30-60 amp with 10-4 AWG wire. You're probably looking at a 3.5 ton unit, so you'll be in the 30-40A (10-8 AWG) range. It might be worth it to get a few quotes from local HVAC companies, and see what they want to install. The companies may have a preferred breaker and wire size, so they may want to redo any work you do anyway. 

Don't forget you'll also need high and low pressure tubing, and control wires between the indoor and the outdoor units.