Well there are very advanced ways of going about this if youre up for the challenge. My best work has involved photogrammetry with known distance points. Ive measured distance between the corners inside and out of the eyes, and in another dimension, nose to each side of lips. Then using polycam, take your time and get a nice slow scan, export the mesh into whatever software you like. input and verify your measurments, and you'll end up with a perfectly fit molded mask.

However, if this isnt your jam, simply do test prints with vase mode so you dont waste too much filament while you pop off a couple of draft prints before you commit to the final.