I made a few different sizes. Here’s what I found:
Different material stretches differently so one persons size may not work for you.
The wider I made my saddle, from lower back to under thigh, the more material bunched up at the bridge ends, making it thicker on the hip area giving more of a squeeze feeling. As of right now, I have found that I like it around 20in without a hem.
I tried cutting the width end that goes under your thighs in a radius for some slack under the thighs. It didn’t seem to help on a hammock end style saddle.
To add some freedom under your thighs the biggest help was cutting the length ends, the ends that the bridge ties to asymmetrical like Justsomedude recommended. The biggest I’ve tried was 3in longer on the end that goes under your thighs than the end that goes against your lower back. This made a huge difference in comfort.
My length ends, bridge ends, of fabric are cut around 40in with the end that goes against your lower back being 3in less. That’s before I add a hem. Your hem style, material stretchyness, and tie off system can change this measurement drastically. It’s a trial and error thing. That’s why I ran out of material before finding exactly how I want my finished hammock saddle.