I am not much familiar with Magnum, but going by size analogy, I will give it a try. Say the manual recommends 12x6 as the largest prop size, then a good starting point for a 3D prop would be on size higher and one or two pitch lower e.g. 13x4. I would give 12x5 a try first. Note, run this prop only after you have completed the run-in with the recommended prop as per manual.

Ameyam