Prop difference?

[cfelton]

Been looking around and now Im confused on what prop to try. Pretty sure I need a 19 pitch but what diameter? They range from 13, 14, 14.1/2,14 5/8,15 and so on. Ive got the old alum prop that probably came on the boat but its not in too good a shape. Its been painted a dozen times and I cant see anything on it. What do you all suggest I try.