What size should I buy for my baby?

Baby Merlin's Magic Dream Sack is a roomy sack design that comes in two sizes:  Small - 6-12 months (approximately 16-24 lbs.) and Large - 12-18 months (approximately 22-28 lbs.).  The 12-18 month size has foot openings for more mobility in older babies. The sizing is based on industry standards for sleep sacks.