I have been playing around with paper sizes.
This is something I saw done with a 12" card square - which made a 6" card. Great, I thought, but too big. Most modern gift vouchers are credit card types that you can add to and spend from, so you need a smallish pocket - or just tuck some money in instead. And this will take you gift and make it special.
This card is made from an 8" square of double sided card, cut and folded so both sides of the pattern show on the front to create 2 pockets. I have made a co-ordinating tag, and tucked it in to the smaller pocket. You can write on the tag as well as inside the card