Size it up
You can print greeting cards of any size, as long as the paper for the project can fit in your printer (and in the envelopes that you plan to use). The easiest kind of card to make is the single-fold variety: Take--basically, a sheet of paper folded down the middle and opened like a book. To make this kind of card, you'll need to print one side of the paper, and then feed it through the printer a second time and print it again.
For this example, let's print on 4-by-6-inch photo paper. When folded into a 4-by-3-inch rectangle, a card of this size fits nicely into small envelopes that are available at any office supply store.