How do you know if your hymen or vaginal hole is too small for sex and has to be surgically opened etc?

Try a tampon. If you can insert a tampon, it is unlikely you will have trouble with sex. If you still have concerns, see your doctor for an examination.
Rarely needed. It is pretty rare that a women will need to have a surgical procedure in order to have intercourse. If you have extreme pain with intercourse or your partner cannot penetrate you should see your ob/gyn physician.