I'm putting together a sig 4 star ARF. Putting a OS 90 FS on it but can't decide on what size prop. Any ideas. (normal Sport flying) Not to fast not too slow. Thanks for your help.

I would recommend that you look to the OS manual for what they recommend for the engine as a start point. After that consider what type of performance or look that you are going for. A low pitch will give better low speed performance, with a lower top end but more static thrust. And A higher pitch will give you more top end and a much higher top speed. I would say a 6 pitch 3 blade should provide what you want and as far as top speed that is what you throttle is for. I try to use a prop that presents the most loading to the engine at high RPM and just use the throttle for top speed.
Regards
Paul

I have used many different props on my 60 size Ultra Stick with an OS 91 surpass ane the 15x4 w apc gives the best verticle and a 13x8 apc is the winner for all purpose sport flying. I have used a 14x6 but it wasn't enough load and a 15x6 was too much only 8400rpm.

Mine flys very well using an apc 14x6 on the os 91fs mounted on my 4 star 60.

try 13x8 first, then experiment!