I own 3 of these jerseys because I like them so well. The fit is somewhat relaxed compared to the typical Castelli race fit. I wear a large on my 160 68 frame. There are some nice features like the extra zippered pocket and the mesh vent fabric in armpit area and strip across the back.
I'm in my late 50's and out side of the occasional century, I ride for fitness and weight control. As cool as those form fitting jerseys look on the Tour de France riders, it's not necessarily my style. The Castelli Prologo is just loose enough to feel good and on a long ride, comfort is everything! My perfect jersey!
I really like the look and feel. I got the large based on reviews and glad I did. I am usually medium but this fits a little snug. I am 5' 10, 160lb. The red is brilliant and the graphics are cool and understated. The jersey is light and good in the very hot Florida summer. The torso and waist section is a bit loose but I think the medium would be too tight for me in the shoulders. The material breathes well and super light. Overall great jersey!