Swim brand sizes

So I recently ordered a long sleeve sun protection top/swim shirt from Speedo to swim in since I'm a bit self concious and I'm surprised over the sizing. I know swim stuff tends to run small but this was a men's medium and it fits me perfectly (I wanted it a bit bigger really) as a 5ft3 9.5ish stone F.

Are there any big trends with the sizing of different brands apart from "just a bit smaller"? I don't have any shops I can go to in person so I'm at the whims on online retailers.