When I make skinny elastic headbands, I make them in the sizes below.
Newborn - 3 months - 13 1/4 in.
3-6 months - 14 3/4 in.
6-12 months - 16 in.
12-Tween - 17 1/2 in.
You can also buy pre-made headbands at www.elasticbytheyard.com
, they sell just the elastic too if you want to start from scratch. I've done both and been happy.