As someone who isn't "thin" herself, I can say that it does not matter overly. I mean of course don't go wearing a size too small and show off uneeded growth. Wear the right size for your body. Miku can be worn by most sizes if done right. What a good cosplayer is, is someone who can show off the character as accuractly as your body will allow. I think that if you want to do Miku, then you should! Just make sure you do the character justice and make yourself feel good too.