When I first started trying to make doll hats and bonnets, I thought “This will be easy. They are just smaller.”
I was wrong. So very very wrong.
Doll heads are just so different than human heads.
Each doll is different. Porcelain dolls have porcelain hair that can drastically effect the fit of a hat, let alone a bonnet. Dolls with soft hair are sometimes easier, depending on how the hair is set. Then there are cloth dolls and papermache dolls.
I have found that even with getting a selection of measurements as well as photos, I need to make a hat stand that matches in size for each doll. Even then, it takes two or three sewing attempts to get a satisfactory fit.
Don't Paint the Cat 