At Artis Styling, bonnet sizes are based on capacity, not head size/circumference. All our bonnets are adjustable (outside of the baby bonnet) for a secure fit. Here are our bonnet sizes and the capacity it can hold.
Kid's Bonnets:
Baby: Ages 0-1
Toddler: Ages 2-4; if your child has shorter hair, this is a greater fit, adjustable, while still having extra capacity for their moving and growing strands. Size up if their hair is beyond their shoulders.
Child: Ages 5-8; A step down from our adult small, this size is perfect for growing kids as their strands continue to grow.  Size up if hair is beyond armpit length.
Adult Bonnets:
Small/Medium (12"): Don't let the name fool you, this size bonnet is mighty! This length is best if you have in a shorter length style, a wig or weave, a low/high braid or ponytail. It still has extra capacity remaining at length of 26" for extension styles. For a smaller size, if you have  shorter hair, I would suggest choosing a child sized bonnet :)
Medium/Large (14"): This bonnet is able to fit a medium to large amount of hair. This size is great if you like more space in your bonnet, have waist length hair, or bulky protective styles. We prefer this size and capacity because the hair is able to be off your neck and back while you are asleep.