Semi-scientific answer: Skiptooth chains are one inch pitch. Measure from the center of the axle to the center of the crank in inches. Multiply by 2. Count the number of teeth on the chainwheel. divide by 2. Add to the last number. Count the number of teeth on the sprocket. Divide by 2. add to the last number. Add at least a couple inches (or more) to account for the fact you must have two ends that will mesh and hook up, to account for the fact that you may have not had an even number of teeth on your chainwheel or sprocket, to account for Murphys law, and to account for the trigonometry I didn't do. Don't forget the master link.
Practical answer: They're all too short. I don't mean that to sound snarky, but it's true. They have been out of production for years. I couldn't buy a new one in 1978. A few years back a CABEr tried to have some manufactured, and the chain manufacturers all noped out of it when they saw the sample. The only way you are likely to get a really long one now is to pay through the nose for an NOS one that 3/4 of the population of this site also wants, and happen to be browsing "For Sale" at the very moment it is listed before the pileup occurs. Anything used and usable probably came from a girls bike, or a 24 or something, and is way too short for most 26ers with full sized sprockets
In my opinion, just run 2 master links.