The different sizes are made by changing the needle size.

Gauge:
On No. 11 Needles: 7 sts = 2 inches, 5 rows = 1 inch
On No. 13: 13 sts = 4 inches, 9 rows = 2 inches
On No. 15: 3 sts = 1 inch, 4 rows = 1 inch