Letter Journals are a great format for doing any kind of art, but especially collaborative art in a round robin style. I first learned about them through Jade Herriman; check out her link for more on LJ's! They are small; about 4" wide by 8" tall. They fit into a business-size envelope to easily allow sending back & forth/collaborating in!
I love the format. I also love collaborative art! I've wanted to get this exchange I did with my dear friend Sharon Borsavage up on my website for some time and today was the day! I have created a New Page under my Art Journals section, called: Letter Journal w/ Sharon B. Please check it out and see what you think!