Hey, Tammie! I make cards this size from time to time- usually when I'm tired of being confined by the dimensions of an A2 card! Anyway, I love the long length of this size card and I think it's fun to play around with different designs! Think out of the box and try something new!
Also, I have also heard of people using scor-tape (or any other type of tape adhesive for that manner) to close off the part of the envelope that isn't being used. I've even seen people punch designs into that part of the envelope- super fun!
Here's a link to a card I made recently to fit in an #10 envelope!
sweetsteph card creations: Market Street January Release SNEAK PEEK!