I'm a big fan of overhang - that is, allowing design elements to extend beyond the edge of the card. Here's an example:
Doing this not only makes the card look larger, it also makes the card a little more interesting. Of course, finding an envelope can be a challenge :)
Here I have the Spellbinders butterfly sitting on the open circular window with the wings overhanging the edge of the card. I must say, the Wonderful Wings set is one of my hardest working sets. The butterflies are a really useful size, and it's wonderful having the shadow dies included.
The full materials list can be found on the DownUnder Direct Inspiration blog.
Thanks for looking!