What size of water line pipe comes out of the wall to the toilet?

normal size is 1/2 " inside diameter. I have seen 3/8 threaded on 50+ yr old homes. Galvanized steel pipes rust and are weakest where threads are, and especially where steel/brass connections are. If threads are rusted out, you will have to change pipe coming out of wall. They do make brass/copper adapters that go from 3/8 threaded to 1/2 copper. good luck!