This rain jacket is made to fit over riding gear. Generally speaking, REV’IT! gear runs a bit on the smallish side and the same goes for the rain jacket. Example: The size XL rain jacket here measures 48” max chest which is just 2” bigger than a normal size XL jacket. And in other brands of rain gear, a rain jacket would normally be at least about 4” bigger than the normal chest. So, I’d characterize the sizing of this gear to be about 1/2 size smaller than most other rain gear.
My advice then… if you ALWAYS wear one size in men’s clothing, then buy that size, but if you are sometimes between sizes, I’d advise you to buy the larger in this one.