Which ever size 2 blade prop you would use for your application (engine, plane, what you are doing with the plane ), either reduce the pitch by 1 OR the diameter by 1" to get "equivelant" performance.

In reality, unless you need the ground clearance, or it is for "scale" appearance, I suggest staying with a 2 blade prop. You have a larger selection of diameters x pitch than 3 blade and the price is less - quite often substantially less, and they are a lot easier to balance.