The skort is the most-worn golf bottom, and extended sizes are where the grading quality shows up fastest.
Everything in the general skort guide applies. These are the differences that matter in extended sizes.
A skort graded properly for extended sizes gets a longer rise along with the extra width. One that was merely scaled up keeps a rise cut for a smaller body, which puts the waistband below the natural waist where it rolls and digs.
Look for a rise measurement on the chart. If the brand does not publish one, that is itself a signal about how much thought went into the pattern.
A stated 18″ falls higher on a fuller thigh than on a slimmer one, because the fabric has further to travel around before it reaches the hem. If you have worn an 18″ in standard sizing and found it short, look at 20″ and above rather than assuming the brand runs small.
Longer lengths also reduce the ride-up that causes most mid-round adjusting.
If your waist and hip fall in different sizes, take the hip size. A waistband can be adjusted or accommodated by a knit band; a skort that binds across the hip cannot be fixed and is uncomfortable the moment you sit in a cart.