Not really a tutorial...
If you know how to make your own envelopes you don't have to stick with standard greeting card sizes. Basically, you don't have to be stuck in a box.
When I was making greeting cards I would make my cards in any shape I wanted because I made my own envelopes. When I mailed out some of these cards I had to put them in a bubble wrapped envelope, but I rarely mailed the cards that I got too embellishment-happy with.
If you don't know how to make an envelope, find one and carefully take it apart. Measure the width and height of every part that will be folded above a fold line.
If you are making large envelopes for a large card, you may have to add another piece of paper to join the two side by side. I would do that before placing the card on top of the paper to measure all parts, then you can cut to shape.
I make envelopes using all kinds of paper too, not just standard cardstock. I've used wrapping paper, vellum, chipboard, craft sheets, brown paper bags, old gift bags, etc.
If you haven't made your own envelope, you should try it. It will save you some money and your greeting card sizes won't have to be so limited.