What works best though (And pretty easy) just measure the part you are doing. Let's say head from the lowest point (chin) to the highest point (Top of head) if your 3d model uses inches then compare inches, if your 3d model is in cm then just convert your inches to cm. Then add 1-3 inches to account for padding and etc (or 2-5 cm)
In reality though you will almost definitely have to build a few pieces over because we're all built differently so I know you'll be doing a lot of modification on your pieces overall, but this will give you a good parting point
