Based on the previous reviews regarding sizing I decided an XL was the best option, though was surprised when I found myself swimming in what most others called slim. I'm not tiny (6'5" ~200) and after ordering the XL an swapping for the L I can confidently say you should pay way more attention to what size width wise you should be and less to the reviews regarding arm holes.
If you're between sizes and concerned whether your arms will fit I would err on the side of your regular size. The vest is designed to keep tight to your body and part of that means ensuring there isn't a huge gap to let in the breeze around your armpit. If you're a 'unique' size then you probably know how much you should adjust from what typical reviews of other items suggest, but if you're in doubt on this one then go for your standard go-to, not the common reviewer suggested oversize.
The family of best selling Crew jackets now has an addition, the Helly Tech Crew Vest. For windy days on the water on not so warm summer days this light weight vest offers protection and style.