There is no standard Cleat size per boat size because cleats can be used for different things, but as a general rule of thumb you should gauge by the amount of force you will have on the cleat and the use. For example, are you Towing with the Cleat or just tying to a dock? You need to examine the load that will be displaced on the cleat. You also need to carefully examine how the cleat is fastened to insure it will hold to the load desired. In our opinion you should always through bolt a cleat and use a backing plate if substantial load will be applied.
Another consideration is what size line will be used, and this can also be a guide regarding the load that may be expected. Bow cleats need to be generous to accept large mooring pendants of up to 1" . Stern and spring cleats depend on dock line sizes.
You will likely want to consult a marine contractor or boat yard unless you are experienced with hardware selection.