What is the proper amoxicillin dosage for a 20 pound dog?

A few days ago my dog was attacked by a pit bull and almost lost the tip of his tail, they bandaged it and gave me some meds to give him twice a day. This happened on Thursday, and it just came to mind that this dosage may be too big and that's why my dog is showing signs of an allergic reaction now. It's 100mg twice a day. He seems to be very itchy, especially around the ears, and I don't know how to check for swollen legs because he is a shih-tzu and has lots of hair. Help would be appreciated