To keep engine loading about the same with different props, I take take the square of the prop length, multiplied by its pitch, and match that factor up with other prop sizes. With this equation, you are moving the same volume of air with each prop revolution for the different prop lengths. To first order, increasing the length by one, and decreasing the pitch by one is pretty close for most prop sizes.

So a 10x8 prop would give me a factor of 10*10*8=800. If I want a prop length of 12 with the same loading, the calculated pitch is 800/144 = 5.56, so a 12x6 would give you about the same engine loading. Note that just increasing the length by two and decreasing the pitch by two gives you the same result here.