One of those in-between options that Angela at Save the Date Originals is fond of is... the curvy invitation. The main card is still about 5x7 inches, but cut into a frame shape. Check it out:
That first example shows off a design with sunflowers; The next one steps it up a bit with color envelopes:
The purple and gray of the printed cards is closely (though not exactly) matched by grey and purple envelopes.
There are as many possible design ideas for this shape as there are with a 5x7 inch rectangle, so if you find yourself feeling like you want to do something a little more fun, this is a shapely option for you to consider.