Here's the challenge thread about Easel cards:
Forums at Splitcoaststampers
The rule of thumb is that your base needs to be double the size of your image panel.
So while the example in tutorial in the challenge link is 4 x 4, you can make any size card if you double the image panel to create the base. Example: say you want your card to be an A2 when folded up, your image panel would be 4.25 x 5.5. You would want your longer piece to be 4.25 x 11 for a portrait layout. Then you will want to score at 2.75 and 5.5 on the base to create the fold to attach your image panel.