I have created a Christmas card containing 2 snowflakes. I have printed the model using Colorfabb PLA economy white, transparant red and green. Printed at 200 dC, 60 mm/s with a heated plate of 60 dC. The dimensions of the card are 110 by 158 mm, which will fit in a standard EU DIN A6 envelope. I did not check on US sizes.
Printing of the model is straitforward. I used 3Dlac to fix the card to the (heated) glass plate, and once ready with printing put to glass plate in the freezer for 20 minutes to easily remove the card from the plate without breaking the card.
The gap to merge the two halves of a snowflakes is somewhat wider than the thickness of the card. This will allow for printing tolerances (expansion on edges), i.e. easily merge the two halves while not falling apart. This should work for most printing processes and filament.
Happy printing and have a nice Christmas!