The one I built, which has worked decently so far...has a 19" pendulum, which is about six inches below the table on which I lay my oversized gut. I'm a pretty short guy (5'8" w/ a 29 inch inseam). I also use a strap that adds about 6-8 inches of length (although not straight down from the pendulum, more sidways as I pull).
You can get a good idea by looking at them on Louie's site but his are certainly wider than most will find necessary. But it's easy to modify for personal use, A big wide body kind of guy needs a wider setup - a taller guy will need a taller setup with a longer swing arm. The important things seem to be to get the axle as close to the hip hinge as possible and to make the base long enough from front to rear that it doesn't tip or walk across the floor when you use it.