I got this handy dandy mailing template at my Post Office. I was asking one of the clerks about envie size and such and he handed me this to take home with me. I use it often! He didn't charge me for it so I would assume you can ask for one free of charge.
This picture shows the inside that has useful information about sizes and such. Notice the slots. The large slot is for measuring depth of larger envelopes and the smaller slot is for smaller envelopes. There are a few useful guides on the template, and some useful information.
Here is a picture of the square envie I made showing how the envie fits through the small slot. You can not force the envie it has to fall through on it's own. If it doesn't, then get ready to pay extra postage. While it did fall through easily right now it probably won't after I stick it another envie for mailing.