I built a SPAD PQHOR and powered it with an OS .25, but it wasn't very stable. Wind has to be nearly non existant. But, then again, it's a SPAD, and didn't have a symetrical wing. For something like a profile or 3d, I would think a .40 size would be more stable. Commercially, I think you'd be hard pressed to find one smaller than a .32 - .46.