In my part of the woods (Wisconsin) horseflies are quite large, between 1/2 and 1 inch but mostly around 3/4. Deer flies are much smaller with a distinctive triangular shape made by their wings when they land.

Both have a very irritating bite, but the deer fly is a small sting of pain that goes away quickly while the horsefly tends to leave a large welt that sticks around for at least a day or two.