The single most important factor in determining the finished size of a knit item is the gauge. Although not as important for flat, one-piece items, it is important when making a clothing item that needs to fit properly.
Before beginning, it is important to make a gauge swatch about 6 inches square using the recommended stitch pattern(s) and needles. If the project that you are making will be worked in the round, work your gauge swatch in the round as well. Wash and block the swatch.
When the blocked swatch is dry, count the number of stitches and rows in the center 4 inches. If there are fewer stitches and/or rows than indicated in the Gauge section in the pattern, your needles are too large. Try another swatch with smaller needles. If there are more stitches and/or rows than indicated in the Gauge section in the pattern, your needles are too small. Try another swatch with larger needles.
Continue to adjust needles until correct gauge is achieved.