BJD head from 3D model - casting, making

Your 3d program most likely has an option to measure your model. This i would recommend first. Otherwise you could load it up in a 3d printer program to check the size, perhaps even scale it. A simple free slicer (3d printer program) is this: Download | Creality 3D
After install you can drag and drop your 3d model (obj or other supported file types) into the main window, select the magnifing glass and it will show the size of the model. I would also recommend 3d printing your head yourself or using some printing service. Just a cheap print so you can see if everything really is ok. Its a big step to go from a 3d model on your screen to something in rl that has to fit and match other stuff. A cheap testprint gives alot of feedback

 

If you have a body then look up several companies' body and head measurements and scale your model in a 3D program. And printing services usually provide resizing options after you upload your stl file.

I had some thoughts about your question. Have you considered wall thickness and neck hole size?

 

SD is super dollfie right? The heights can be found here: ドルフィーについて | ボークス公式 ドルフィー総合サイト | 株式会社ボークス
Insert a silhouette reference image into your 3D program and scale the image to your doll’s height. Then scale the head to roughly match the size of the head in the image.