In my experience, I've had to guess-and-check with various tires to figure out which ones will clear, because each tire model has a different final tire width based on the compound thickness and how round it sits when pressurized. Cervelo's FAQ seems to agree that it depends on a lot, so it might be best to order from a shop that has a good return policy.

I'd say up to 25 or 28 but go to your local shop

buy a single 28mm first and test it out on the front wheel, if it doesnt fit, use it on the back wheel