I have a 60 size Hog Bipe with an OS 91 2-stroke on it. What size prop do you suggest? First time to fly bi plane.

I would put something on that will give you about 10 - 11 thousand RPM at full throttle and see if it works out. Then, after flying it, and especially landing it, change the pitch/diameter to meet your needs. Just remember that shallow pitch is like changing gears on a car.. lower gear (lower pitch) less speed, more climbing power and easier landings (slower) and for the tail draggers, this can come in rather handy, if the plane will handle lower speeds. I'm by no means an expert, more of a novice, but that's what I usually do and find that it usually works out ok.

I would start off with a 16x6 on that plane. I fly mine with a 15x6 on a Saito 100. Great vertical and it will slow down nicely.

2-strks like to be run in the higher RPM range as this is where they make their power. 4-strks don't like to be run over 10k as they make their power at a lower RPM.

I also recommend the APC 15x6~8 or a 16x6 if it can spin it.