Hi!
11x6 is just fine! 12x4 is even better on a High winged trainer. Just use throttle to control decent!

The first thing to do when flying a plane is to check the C of G ...up high. The goal is always, regardless of plane typeto have the C of G placed so that the plane will stall when the speed is reduced to a near stand still and at the same time have full up elevator.
If the plane just mousses forward at that point the C of G is located too far forward. -Simple as that!