Here's some of the envelopes I made. They aren't standard size. More like "thank you" size.
This has inspired me to make some stationary gift sets for my friends and families for the holidays. As for the Dr. Seuss envelopes, I think I might put sets in as a freebie to my customers. I'll have to make custom note cards to go in them, but hey, any excuse to get crafty is fine by me :)