Kigurumi Mask Sizing Options
Many makers will offer a range of sizing options, this is often based on the size of your actual head, but also the proportions you are trying to achieve.
A younger character or chibi style character will typically have a larger head shape compared to the rest of the body, so a larger mask size is often more appropriate. Similar if your body is on the larger side, a larger mask size may be more appropriate to achieve a more balanced look, its all about proportion play and working with what you have.
Opposite can also be true, too large a mask may look out of place, somewhat 'bobblehead like' on a smaller body.
This section is under heavy development.
To be added: examples of each of the mask types.
Maker Sizing Options
Most makers these days use 3D printed masks, which allow them to highly customize the mask to the customer's specifications. These makers often can offer a wide variety of sizes.
Makers that do semi-custom or "generic" masks, or who are still using traditional mold making will typically have a much more limited range of sizes available. Sigma and GKO are two such makers whose main offerings are limited to a few sizes. Custom options are usually available but at a higher cost, longer wait, and other limitations. This doesnt mean they are a bad or limited maker by any means, its just a matter of what they offer and what you are looking for.