FTM binders from Underworks?

Ok I'm gonna start binding my chest but I'm confused on what size to get. I want to get the tri top binder. I'm like 30-31 on the bottom and over the chest about 34 I guess. I don't know if I should get an extra small or small. I been looking at binder reviews on YouTube but I'm still unsure. So I would appreciate it if you gave your opinions on whichever one I should get.