Usually size small (medium, large, me.) In other words, in your example, if you're knitting a small whatsis, knit 19, purl 1, knit 39 and purl 1. If you're knitting a large, knit 23, purl 1, knit 46, purl 1. It helps to go though your pattern and highlight the appropriate number of stitches for your size.
Sometimes you'll see:
Row 3 K1, yo, k2 tog, k to end.
Size Small: Row 4: P2...(and a bunch more directions)
Size Medium-Large-XL: Repeat Row 3 1 (2,3) time(s)
In that case, with a size small, you go on to the next part of the pattern; with medium, large and XL, you add repeated rows to add width or length. Once again, it saves a lot of frustration if you go through your pattern first and highlight the directions for your size.