Hello!
If you use the old gray Graupner props they are not particularly good...not compared to APC or the new black Graupner Cam-props.
11x6 is a good size for a 7,5cc engine (.46) but do not use Master A props... they together with the gray Graupner props are the worst props on the market as they are loud and don't pull as good as APC props.
As for a prop for a 15cc engine...well a 14x6 APC is a good prop/size but for a slowflying airplane a 15x4 or 15x6 APC could be used.

Regards!
Jan K
Sweden