I'm 5'10", and made a gathered end out of 12' of ripstop. At the moment the ridgeline is 116". I am still playing with RL length, but 110"-116" gets me an almost perfectly flat lay.

It's all personal preference. My last DIY was 10' with a 108" RL, and I also have a HH Explorer (just about the same dimensions). Both are comfortable, only problems I had involved CBS, but not as nice as the 12' monster.

Try different sizes, find what you like