Ah. There's the rub. A propeller generates more thrust at the tips, and the longer props have more tip (relatively speaking). But if your're cutting grass as you try to rotate onto the step for takeoff you've lost any advantage.

I run 11X6 on my TT Pro-46. Nice engine. I have an assortment of props I carry in my field box (all threaded on a 6" bolt) so I can swap out props and see what works on an airframe. I have been surprised at times. As a rule of thumb: if you move an inch in diameter move an inch the other way in pitch (i.e. 10X6 => 11X5.). I have a model with a ST .51 that is a dog with a 10x8 but has lots of vertical and slows well with an 11X7, and the 10X8 is not noticably faster. You just have to play around.