I started a job two weeks ago and was worrying about the same, except that I did not need to wear a uniform. It made me very nervous, because like you, I would want to go full time for the first time, but had no idea if I would pass or not.
My advice is to just be clear on your name right from day one. Even if they would doubt at seeing your appearance, your name should make it clear that you're a guy, and would like to be addressed as such. Don't be insecure and stand by the fact that you're a man, and should be treated as a man. I would also just go to the men's bathroom (if they have separated ones) like it is no big deal. If you doubt, people start doubting too.
I don't know if you've told your boss about your 'condition'. If you haven't, it may be a really good thing to let them know before you start. Especially if you haven't changed your name yet. My mistake was to not let them know, so on the first day, they used my girly name. After I told them, they switched and the mistake was barely noticed by my co-workers and things went really well after that. But it was still very awkward and could have been prevented if I had notified my bosses.