i love castelli im 5'9 175 lbs i have bibs in large size but i feel that i could use mediums,, in in the jerseys size i thought i was a large but i switched to medium and i'm glad i did.
so 5'9 175 lbs Medium in castelli sizing

plus the castelli bibs feel like they stretch out a bit mainly with the bibs. jerseys also stretch out a bit