Castelli Size

Hey guys, i was thinking of buying the Castelli Mortirolo 4 Jacket, but since i never purchased clothes from Castelli Brand, i'm not sure what size to choose. Some people advised me to go for a size larger. Im 5'7", 123 pounds, 29.5" waist and 33' chest( Yes, i know, im very skinny) and i usually wear small on everything ( in cycling). What do you guys suggest me, small or medium?
 
I have just got hold of a long sleeve Gabba, best piece of clothing I've ever tried. I'm 5'11", 158 pounds, about a 30" waist and 38" chest and I have a large Gabba which is race fit but not uncomfortable and I could get a baselayer and a jersey under it if I wanted. Based on that I'd think you're probably a small if you want race fit, but try it on if you can (any friends got one?) and buy from a place with a good returns policy.