I personally don't have the Just because cards cartridge. I do however have Wild Card cartridge.
With the wild card, I see how big an envelope can be made. Some can be bigger than others depending on the shape of the card. If it fills up the 12x12 (you can tell by the LCD screen) at lets say 5.25", then that's the size you ought to cut the card. You don't want your card to be bigger than the cricut is able to make the envelope. Make sense?
Once you know how big an envelope can be made - again, let's say 5.25" - check to see how big the card will be at that size. Knowing you can size down if needed...
Or, size the card as big as you'd like and purchase a We R Memory keepers Envelope Punch
To use an existing envelope, what size are we talking about? A typical A2 "invitation envelope" size (fits 4.25" x 5.5") ? It will all depend on the shape of your card. However, it's way more fun to use your own papers to make personalized envelopes using the cartridge function or an envelope maker. Plus, you have more freedom to vary the size of the cards.