Wow what a question! You must first know what you are starting with! What is on the boat now? It will be stamped on the prop. Prop sizes are given in 2 dimensions, diameter and pitch. So a 13 x 13 is 13 inches in diameter and will go 13 inches through the water in one revolution. Second you must answer the question of what do you do most with the boat? Skiing usually demands a 3 blade with a pitch of 13 inches or more, higher speeds. Wake boarding is usually less with a 4 blade and less pitch. Determine what you do most and then call one of the prop manufacturers and see if you really need to change. As a rule I only have Stainless Steel props on my salt water boat (its is a fishing boat not a ski boat), but keep the brass alloys on the ski boat. If you ever hit something the brass is easily repairable and because it is a softer material than stainless it will not transfer the force up the shaft to the engine and boat!