A while ago I had some 5" x 4" sleeves made to fit Talisman character cards. They also fit all of Talisman's large cards, like Endings, Toad cards and NPCs (and fan-made rules and information cards!)
Descent also used that size and now it seems to be the Terrinoth standard with Dungeonquest using the same format.
The problem with the ones I had made is that they are only 40 microns thick, so FFG's standard of 100 microns would be excellent for these.
20 - 25 per pack would probably be okay, and keep the cost down a bit for people.
I'd buy a load of them in a heartbeat!
Sometimes I even update my Facebook page for the site!