You don't need to know the size, partly because it varies prop to prop. Your chosen supplier will require details of your boat and engine and will then supply the correct size. Just to give you an idea why it is important to take the supplier's recommendation, my 33 had a 16*13 fixed prop from the factory but the Flexofold folder size is 16*11.
Suggest you contact Darglow Engineering as they supply FlexoFold which are arguably the best folding props.
