My son is 5 months old and his head is pretty big.  I ordered the larger size and it fits him well enough, even up to 2 years old I would say.  Bands are adjustable and I'm fortunate that my baby never took them off.  I love these sunglasses!!!  They are the cutest thing ever, and my son received so many compliments at the beach with them on.  I like the polarized ones too, so that baby could see better with glares.  I only wished that Amazon would carry a larger variety of these in different colors.