RE: Rules for 2 stroke engine's break-in
If the break-in calls for a 10x6 then that is what i would use. After break-in, an 11 will probably be ok. The only drawback might be a delay in the engine winding up. As far as mounting location, I would leave a minimum of 3/16" from the rear of the prop to the sides of the body. You will probably have some thrust angle, if it is built into the firewall like most, just measure on the short side. Balance will also affect some of the measurements here, you should have about 3/8 to 1/2 inch of adjustment to aid in balancing.