For adult sweaters, finished bust sizes range from 36 to 54 inches.
So if, for a modified drop sleeve sweater, which is the page I happened to open to, you look at the bust size at the top. Then you go down to yarn requirements for whatever gauge you want from 3-7. It tells you how many to cast on for that size in that gauge and tells you how long to knit it.
Then you shape the armholes, shoulders and back neck and then gives you a choice for a crew, v, etc.
There are also children's sizes.