Invitation sizes. Also on this page: envelope styles/sizes.
Card Sizes http://littleharborpress.com/cardsize_info.html
How to Make Any-Sized Envelope the Easy Way
A chart to help you create the perfect box with the Envelope Punch Board from Stampin' Up! : Beverly's Blog
Formula for making your own envelopes for any size card.
DIY Cash Budget Envelopes: Free Printable | Learn how to create your own wallet-sized envelopes using any paper or scrapbook paper. | @thinkingcloset
Box Buster. Use this online calculator to figure out how to use the EPB to create any size box.
I'll have to try this type of shape. My guess is it would fit into a legal size envelope?
Pop-Up Box Template...fits invitation size envelope.