2-3 sizes small for me. All my mountain bike stuff is the same size I wear normal clothes in but Castelli road stuff I need a 3XL for jerseys and 2XL for shorts. I wear 34" for jeans not fat at all, their stuff just runs tiny.

My fiancee wears the smallest size possible for regular clothes. Extra small, petite, etc. She wears a MEDIUM in Castelli