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I got a Castelli Gabba RoS jersey (or sleeveless jacket???) mail order as it was on sale.
I used Castelli's size guide to choose the size but when I tried it on, it doesn't feel right.
I'm assuming the material needs to fit tightly against the skin to keep you warm but it seems way too loose everywhere except across the top of my shoulders.
Do I need to go down a size (or two)? I'm 191cm and 89kg and went for XXL...

Jacket/jersey? I'd always wear a baselayer under it. Theyre certainly a heavier grade material than most jerseys I can think of.
I have to size up two sized in castelli. Large in Rapha, XXL in castelli.
Maybe it's a size too small?
Do I need to go down a size (or two)? I’m 191cm and 89kg and went for XXL…
Maybe!?
I have a Gabba, older generation, and at 183cm and 80kg I'm in a Medium for a 'race fit'. Excellent bit of kit for anything below 10C. Even goes sub-zero with a base layer and some Castelli arm warmers.
@tall_martin: it's definitely not too small.
Anyway, Castelli do free returns and even at sale price I want a proper fit so it's going back...
Mines tight across the back of my shoulders while i stood up, much more comfortable when you're on the bike though, it's weird.
I concur, they have excess fabric in the back of the shoulders, which should be comfy when leant over the bars. Feels a bit weird when off the bike though