I have been flying an 88 inch, 1/4 scale clipped wing cub with a ST-90.
Plenty of power and speed with a 13 x 8.
I just put a Brillelli Toro on it and am planning on using a 16 x 8.

I 'm thinking a 6 pitch prop may be to slow, and APC doesn't list a 7 pitch prop in 16, 17 or 18 inches.
Of course it is a Cub and a 6 prop may be very scale.