Hi Gael,
just a note on the plan to work out and sizing -- everyone is different, of course, but if you're not on t working out may not make you a lot bigger in the areas that the binder would compress. I'm definitely a hard-gainer even by female standards (also pre-t), a rather skinny type. I've been working out hard for the past three months and I have definitely gained some muscle and I'm happy with the results -- but it's mostly on my arms and thighs, so nothing that would make a difference in the size of the binder. I'm definitely stronger now, but not much bulkier

. I gained about 2-2.5 kilos (I had to literally stuff myself with calories, that was much harder than the workouts themselves XD) and my body fat increased a little, but not significantly. I'm now roughly 64.5 kg at 6 feet.