just finishing up a hog bipe with a magnum 91 4 stroke for power what do yall think about prop sizes for this combo?

I have a Magnum .91 4-stroke. It seems to like 13" props for some reason. I would probably start with about a 13x9 or so. That should get you real good performance. I run that size in my Extra(pictured in my avatar), and get totally unlimited flight. If it's a new engine, I'd go 13x8 tuned to the rich side. My engine with this prop tachs at about 9300+/-, I can get it to 9500-9700 easily by tuning, but don't need to push it like that to get unlimited. Good engines though. I like mine, it's good value for the money I think.

I have a Saito .91 and I run an APC 14x6 prop. Ample power without leaning out the engine (still running it rich). I take off at 1/2 throttle for a gentle climb and was able to pull out of a stall with full throttle (and some knocking knees )

Bob aka Nuker

I would stay as flat as I could.