I'm a big guy, I won't lie about that: 6'5", between 230 and 240lbs on a good day. I bought the XXL sized rashguard to go with some Hurley boardshorts, with the expectation that it was going to be a bit snug, and yeah, it fit exactly how I expected it would. If you've never owned a rashguard before, keep the snugness in mind, and buy a size bigger than you normally would, if you don't want it to be constrictive.
First-time buyers need to keep in mind that these shirts were originally intended to be worn by surfers under full-body wetsuits, to protect against friction rashes (hence the name, rashguard). This is why they're so tight. People have started this trend of wearing them to the pool or beach because of the UV protection (this particular product has an SPF rating of 40+) that doesn't wash off, and it works well for that. I wore mine all day at the beach, with no sunscreen on under it, and didn't get burned at all in any place the shirt covered.