I believe I was an a b or c cup before binding. It's not "comfortable", but it doesn't really hurt (at least not continuously). New binders hurt as they rub my armpits raw. I'm very uncomfortable with my body and have a hard time sleeping so I tend to wear my binder for about 20 hours a day (at least). I do fold my binder at the waist to keep it from doing whatever it wants. Sometimes the seems dig into my waist (this can be 3 or 17 hours into my day) and that ->-bleeped-<-ing hurts. My breathing is restricted which is why it's uncomfortable. I tend to pass out before or instead of properly falling asleep so I have slept in it. I wake up wheezy which leads to a major cough, but not real pain. I know it's bad for me so if I do pass out in it I try to allow 30 minutes of sleep or forced waking uncomfortableness with with my binder off that day. Oh also, in the summer the heat and moisture form underside of things can make my chest very itchy...also wouldn't call it pain, but don't have a problem calling it torture...
I'm 120 lbs and wear an extra small, double front compression shirt from Underworks.