Jake,
Yeah, I'm guilty of being a knifemaker. 15 + years of experience at it part time. Was full time for awhile, hope to go full time again sometime in the future.
To answer your question, most knife scales will be 3/8, 1/4, or 1/8" thick. Pieces or blocks will be 1-1 1/2" thick. All will mostly be 5" long.
I've looked at some of my suppliers for knife supplies catalogs and have considered buying dymondwood pieces off them to cut into pen blanks.
Knifemaking is a very interesting hobby, but not one that you will learn as quick as making pens. Start with a quality kit knife and simply attach the handles and work up from there. Or, you can be a stubborn cuss like I was and jump in with both feet and learn how to grind, heat treat, temper, and finish from the raw material. Either way, it is a very satisfying hobby.