These little 3 x 3 notecards were cases from the Stampin Up leadership event this year. I also have made matching envelopes with my diagonal score board.
Here is a PDF file with all the measurements for different size cards.
And here is how I made the 3 1/4″ x 3 1/4″ envelope:
Cut a sheet of paper 5″ x 5″
Then score on the diagonal at 2 1/4″ on each side
Cut out the corner notches as shown
Corner round the four corners