Ok, I've got the Golberg Tiger 60 with a Magnum xl-80rfs on it. Currently running a 13x6 prop. If I go full throttle and pull vertical, the plane will climb, then slowly slow down then stall, I want the plane to be able to climb vertically(unlimitted), so I'm being told I should go with a .91 size 4 stroke for the added power. I just found a web page that recommends a 12 X 8 prop for my motor.... will this help climb performance, or make it worse for my plane? Or should I go with a bigger motor? I was just told at my flying club to always go with a bigger motor than what the manufacturer recommends, Golberg recommends a .70-.80 4 stroke for my plane, so I went with the .80, the guy I spoke with told me to go with a .91 and I'd have great power for all future aerobatics....
If I go with a .91 4 stoke, any recommended prop sizes? any advice would be great. Thanks

I dont run magnum engines so Im no expert on this subject but I would think the prop you are running is fine. Thats the same prop I run on my saito .72 for hovering (usually). It sounds like to me that you are a little bit underpowered as far as vertical goes. Knowing that you are running a 13-6, I dont think that a prop change is going to make a whole lot of difference. If you want to try a prop then go with a lighter, wider blade prop. I would say a 13-4w APC (these are about 15 bucks) or a 13-4 Zinger wood prop (around 5 bucks or so). I say the wood prop would be your best bet due to its light weight producing higher RPM with a Low pitch.
If you do decide to change engines I would go with either a Saito or a YS in the .90 range ( I think both are .91). YOu might also check out the new saito 1.00. I believe its lighter than the .91 and produces more power. When you check the price on these you will be stunned at first but you get what you pay for in this hobby. If you choose to go 2 stroke then I would go with something like the MVVS (it offers more bang for your buck IMO).