Click the photo to view the full size; there are [crude] instructions there as well as a download link.

It prints on 8.5 x 11 paper, which is standard printer paper. You can easily print larger if it’s in that same proportion.

The best way to go about making this is to print it out and glue it down to a thicker, heavier stock of paper. I used rubber cement (dry mount method) as my adhesive and finch paper. This will actually make it more sturdy and less fragile.
When it comes to folding it, however, thicker stocks of paper don’t fold as willingly as regular copy paper. What you do is take your straight edge and either use the back of your razor blade or a metal paper clip and score the paper (art side down) where you’d fold it. Doing this will allow the paper to fold much easier without damaging the art.

However, if you can print it on a heavier stock without any issues, go for it. 

You may use this for whatever purposes, but do not mark it as your own. I spent good time getting this die-cut to actually work properly.

I built the entire envelope myself in Illustrator.
When it is cut and folded, it will look perfectly like the envelope featured in the series.