Some manufacturer's size ranges vary significantly.
This chart provides general height and weight guidelines.

Size Weight Height
0-3 Months Up to 10 lbs. Up to 22"
6 Months 11 to 14 lbs. 22" to 24"
9 Months 14 to 16 lbs. 24" to 25"
12 Months 17 to 20 lbs. 25" to 27"
18 Months 21 to 24 lbs. 27" to 30"
24 Months 25 to 28 lbs. 30" to 33"
2T 25 to 28 lbs. 33" to 34"
3T 29 to 32 lbs. 34" to 36"
4T 33 to 36 lbs. 37" to 38"
4 35 to 38 lbs. 39" to 40"
5 39 to 41 lbs. 41" to 43"