Rashguards are supposed to fit tightly on the body to avoid excessive rubbing against the skin and causing rashes!

I'm 5'11", 155lbs and a medium fit me perfectly. A little difficult to remove when wet, but still a good rash guard.

Don't buy a size larger than normal unless you plan on wearing it like a tshirt.
This would make an expensive tshirt.
However; as a rash guard, it's well priced.