I see this issue all the time with beginner cosplayers. They get a fancy new 3d printer, download a cool helmet and print it, and it doesn’t fit right.
In this video, Tina Ross walks you through the process of getting those helmets to fit your head perfectly. She covers how to measure your head, but more interestingly, she walks you through the process of using a head model you can find on thingiverse.
The process she uses is largely to combine the pieces in meshmixer, then scale it in the slicer. She stops there, but keep in mind this is just how you would find the perfect scale factors to increase. if you still wanted multiple parts, you could write down these scale increases, then just scale up all the individual parts by the same exact amount and BOOM, you’ve got the perfectly sized version of a helmet file.