I think they run small (I'm normally a size XS top but in these a S is fine) and the sleeves - for those of us that are lanky - are quite short. I would imagine for someone with a long torso these could be a hard fit too b/c I find them also not really long enough to stay tucked in great - especially if your breeches gap at the waist when you ride. I bought one shirt and love the coolness of it but wouldn't buy again b/c the fit is just...NQR.
We sell a TON of these shirts. They run a tad on the smaller size as they are fitted. Great shirt with an SPF of 50 and totally keeps you cool. Many new colors available including white with a black collar this month.