Jason,
The extensions should be 18" if I remember correctly. Also, there is a BIG error in the manual about the interplane struts. It is very confusing, and on the web at Hangar 9, they posted a service bullitin about it, AND IT WAS WRONG TOO!!

Be careful to check the lengths of your interplane struts when installing them. One is only about 1/8" shorter than the other. If you put the wrong ones on (front on the back and vice versa) you will twist your wing!

Check the wing incidence after installing the interplane struts! I have a feeling some of the reporting stalling characteristics were due to this. I made sure everything was square, and she flys straight and true without any bad tip stalls.