This card measures narrower and longer than any envelope I had on hand.  What to do?

Make my own envelope!

I used a 12x12 inch scrap piece of cardstock, and made it in a regular envelope fashion but 

adjusting for the dimensions. I made a slight error in folding on the right side, so I 

covered that little mistake with a strip of extra paper and a border punch. 

I do not have special envelope glue, so I glued it carefully shut (but not to the full length of the flap) 

and to compensate I place a cut-out of a stamped flower image on the tip - to act as a seal. 

And off it goes.......to my lovely Aunt Elsie.