I should wear a small for proper chest reduction, but since I have asthma and scoliosis, I went with a medium and only wore it a few times a week at most. I've always had large shoulders and a wider ribcage for my stature, so looking back on it, I don't think a small would have ever fit for that reason. After being on T for a short period, my ribcage expanded some more, and I think my back muscles grew, too, despite not working out my upper body. I probably stopped being able to wear my binder at all a month or so into T. Last time I tried it on, it was super tight, but I figured I'd bear through it for the day out with my friends. I didn't even finish getting dressed and I couldn't breathe and had to rip it off the same way I used to when I would wear it for 8 hours at a time. So I can no longer bind, even on rare occasions, hence one of the many reasons I need my top surgery so soon on T. I'm pretty sure even if I wanted to waste the money, a large wouldn't conceal my chest properly at all (even the medium didn't look great on me).
It's amazing how such small growth changes can drastically affect your size.