I got New Buff color in XS which is the same size as my white one, and I got the Coastal Blue color in S since they were out of XS. To be honest, the XS and S fit exactly the same. Maybe S is a tiny bit longer, I don't know. They really look identical to me.
Here's the coastal blue in S:
And new buff in XS:
Totally recommend these shirts now that they are on sale for such a good price!