I started looking into long-sleeved rash guards to avoid sunburns and also to avoid applying absolute tons of sunscreen to have the same effect. I haven't had a chance to use this in a pool yet but I wanted to comment on the size. I'm 5'4'', 130 lbs and normally wear between a size small and medium top, and a 30F bra size. I purchase this shirt in a medium and it's quite form-fitting. The chest area is a little tight, and I'd probably go with a large next time. It remains to be seen how it behaves in water over a bathing suit though, but it does run small.
Haven't worn it in the water yet, but so far very happy. I'm size 10 and D cup. I ordered Large. Perfect so far
I love my rash guard! Fitted, but comfy, not hard to put on at all. For sizing reference, I am 34 DD, 28" waist, broad shoulders, and the large was perfect for me. I feel like it would also look great on someone a little thicker than me as well, but if you are a bit smaller than me, maybe opt for the medium. The colors of the rash guard are true to the photo. It does not have boob pads, so keep that in mind, because you might want to wear a bikini top underneath it.
It was a nice shirt that had no issues, but I bought a larger size based on previous reviews stating that it ran small. I'm a size 10-12 on my top half, 36 C, so I bought the extra-large. It was too big to swim in. I wanted this shirt to work so badly, being that I'm very very pale with a strong history of skin cancer in my family.
I ordered the recommendation of other reviewers, but this was a bad idea. It made me look like a box. This runs true to size. That said, it's a very nice top, sleek, quality material. Just not for me. If you're busty/top-heavy I recommend the slim fit instead.