When working with a pattern where size is important, such as for a sweater or other piece of apparel, it is important to check your gauge before starting. Even when the same size hook and thickness of yarn are used, the finished size will vary between different crocheters since everyone holds the yarn at a different tension. That is why most patterns will have a gauge that shows how many stitches and rows per a given number of inches there should be.
Gauge: 14 sc and 16 rows = 4 inches.
To check yourself, crochet a roughly 6 x 6 inch square in the stitch that will be used in the pattern; in this case, single crochet. Once the square is done, lay it on a flat surface and using a measuring tape for accuracy, count first the number of stitches in 4 inches (or the number of inches given in the gauge), and then the number of rows.
If you have more stitches or rows than the gauge, try using a slightly smaller crochet hook. If you have less stitches or rows, try using a slightly larger hook.