I haven't actually done it yet... I just figured out how to do... I am in the process of building it now... Here is what you will need to do and hopefully it will work for you:
1. Navigate to to this folder in your moodle installation: mod/certificate/type
2. Create a new folder in this directory called "wallet".
3. Copy the certificate.php file in the letter_non_embedded to the directory you just created.
4. You will need to modify elements in this php file to get the position and size right for a wallet card. Get to know the contents of the file and things should start to become clearer as to what you need to modify.
What this approach will do is allow the student to print out the wallet sized certificate on a regular letter sized peice of paper. They just need to cut it out and laminate it if they wish. In our design department they are making a mock-up with a dashed line to cut along.
5. Lastly, you will need to add a line to the language file of the certificate plugin to display the new certificate type correctly in the drop-down selector when you create a new certificate in the course. Navigate to mod/certificate/lang/en/ and open certificate.php
6. On line 164 you will see the default types included with the certificate plugin. Add a new line under 167 and put this there: $string['typewallet'] = 'Wallet';
Save your work and you should be good! Hope that helped, or at least got you on the right track =)