There are a lot fewer relevant head sliders--your nose and mouth don't affect how hair fits, and does anyone really make his or her skull skew to one side or the other?--so the combinatorial explosion clothiers and avatar makers face doesn't bite quite as hard. I trundled over to Magika (one of the places Busted magazine recommends on its page for potential models) and looked around... and was blown away.
Mesh hair typically comes with a number of sizes in the package so you can pick one that works for you. As with anything else, try the demo first, to make sure you'll be happy with what you buy. Large worked fine for me.
Now, there is one thing that I wish for: if you look at mesh hair, you will find a lot of it has hair that falls forward over one's chest. That leads to obvious issues for those of us who use breast attachments. I hope that hair makers will take us into consideration and offer versions that accommodate us. For now, though, I'm very happy with the mesh hair that I can use.