I can sometimes wear a Small jacket/blazer, but this one in a Medium is snug--I won't be wearing it buttoned.  The sleeves are also tight, more like shirt sleeves than jacket sleeves.  This blazer would work best over a tank top or shell, or a knit shirt with close-fitting sleeves.  I couldn't wear a woven shirt with 3/4-length sleeves under it because the shirt cuffs made ugly bulges under the blazer sleeves.  On the plus side, the fabric has a very nice drape & feel, and the garment is well-made.  The blazer is very comfortable, just the sort of thing I've been looking for to wear to work (my office is fairly casual).  I like it enough that I may try a Large to see if the sleeves are any bigger.