Most report that a .70 four stroke is plenty for this plane. You'll have lots of power with the Saito .91. You can swing a 14x6 or 14x8 (although some report that a 14x8 is a bit much) with that engine. I wouldn't go smaller than 13x8. You might want to experiment with 3-blade props and see how you like them. In 3-blade props, I'd probably try a 12x8 or 13x8 and see which one your engine likes best.