You should try to prop for about 6000 rpm static. That can be achieved ith 24x12 up to 26x8-10
For three bladed props, 22 x10-12.
The large 26" prop should be of the narrow blade type.
I turned a Menz-S 24x12 at 6050 rpm using a Krumscheid 50-70cc pipe without tuning the length for best rpm.
Look at the power curves on [link=http://mvvs-nl.com]my web site [/link] and you wll see what rpm are best suited.
I hope this helps.