Well Supertigre's instructions, brochures and stuff recommend a 22x8 or 22x10 prop for a glow engine. Thus a prop around the size of a 24x8 or 24x10 prop for the diesel sounds about right to me. I think the 20x10 prop would be too small on the engine. if you have to screw in the contrapiston almost all the way that would also imply that the prop is too small too.
But yeah buying a nice assortment of extra large props gets expensive real fast too.