Working in St st, inc each side every 3rd row o times, every 4th row 13 times, and every 6th row 0 times. Work even until piece measures 16 3/4 inches from beg , end with ws row. I did this and the sleeve measures 21 inches and I haven't even begun arm hole shaping. I knitted 3 rows and 4th row made the increase then knitted 2 more rows, then continued in this manner 13 times.
Just what Salmonmac said. This is noted that way on multisize patterns, usually for infants. If this is a multisize pattern, you might find it helpful to print the pattern out and circle the directions for the size you're doing. It will help so you don't get confused later on in the pattern.