Weight of your heli with full payload would be helpful. We really need more info to help. I fly 2826 with 14x6 props on my Octo with 60amp ESC on 5s / 11,000mah. No issues at all hovering at 50% with an AV200, 5d and prime 50mm. I think I could run 15" on this setup. It could easily handle 15" props on 4s although you may consider more headroom on your ESC. Have you run eCalc yet?