I realize I'm blending forms of yaks, but... I'm 5'8" and 140. The sizing guide for a greenland SOF is 3 x(width of your arms streched out to your sides). This gave me a 16 ft yak. Mine might be a few inches shorter, but I've been very pleased with this size. The width is your hips with your fists on either side. I ignored this and made a 24 inch yak. I'm making one that actually fits the criteria now. If you are making it purely for rolling then it calls for a half fist on either side.
This may not correlate directly to strip yaks, but its a good start.
Concidering I'm your size, I can't imagine an 18ft yak making sence.
Scott