I've been playing with the new Envelope Board from Stampin' Up! and I love it. Here's some of the envelopes that I've made. You can make 66 different sizes using the board.
Here's the board. It's only $19.95 so let me know when you want one. The directions are printed right on the board so you can't lose them. And you have two choice when deciding what size you want to make. You can see the card size and you can see the paper size. So if you have a card you just measure it and find the size on the board and it will tell you what size paper you need. But suspose you have a piece of paper that you don't want to throw away. You can measure the paper and then see the size on the board and it'll tell you what size card it will fit.