Here is a fun, new design that is circulating the internet craft community. It is called a "Pop-Up Card Box." When the card lies closed (flat) it measures 6.5 inches square. Not sure how I would mail this card tho. Maybe in a bubble mailer.
Splitcoast Stampers has a tutorial here and a video tut here. Thus, I am not going to give you the instructions. The instructions are clearly presented. However, if you have a Pazzles, I can give you a WPC cut file so you can whip it out on your machine.
I have two WPC versions of the Cut File.
Pop Up Box Card KISS Cut Version
This File has the red lines to be cut as Kiss Cut lines (perforated) for making the folds.
Pop Up Box Card EMBOSS Version
This File has the red lines for making the folds. These lines are made using your embossing tool. I find it slick to use the embossing tool for scoring. You do not need to use the embossing mat. Place your cardstock on the cut mat. First emboss the red lines, then cut the black lines.
Another piece of advice I have, is that you might try decorating the inside "layers" before attaching them to the box. I have not tried that, but decorating the layers already attached was a challenge.
Enjoy the pictures and have fun making your Pop-Up Card Box.