You will need the original size as a starting point, it avoids expensive guesses
Number will be stamped on the prop in the form of (Number) x (Number), this gives you the diameter and pitch. It will be something like 13 x 15 etc.
Props can be fixed easily and cheaply if you are happy with the size of the current one, send it Steel Developments, costs about £40 and it will come back like new. They also sell new props as well, buying a spare prop is a good idea and it reduces postage if you buy at the same time as a repair.