Thanks so much for this great theme, Jo! I write lots of letters, so I make lots of mail-art envelopes. This week I'll definitely make some more. This is a semi-random selection from my archives, some of which have rubber stamps, cloth ribbon, &/or plastic on them as well as paper. All pictures show both front & back. They're 6" x 9", 15.2 x 22.9 cm, except for the last, which is 4" x 6", 10.2 x 15.2 cm. I'm sure y'all recognize the Klimt: 2nd one for Tim, 3rd one for Danielle. I don't know if these are all collage, unless you count the stamps.
Now you know my address, so if you want a long letter in a 6" x 9" mail-art envelope, just write!