The 7 x 6 is good for a safe start.
In general the typically-recommended prop for a .15 is a 7 x 6, but it under revs the engine and does not give the highest speed.
Maybe you will scuff the prop on takeoff. Then you can cut it down and balance it an probably go faster.
The winning speeds in Perky are done with RPM in the 25,000+ range. Do you know the RPM of your engine?