Here are some examples of greeting cards in standard frames.
A simple frame is all you need to make a small reproduction into a lovely statement.
Joan in the Walnut Canopy Postcard in a 4" x 6" photo frame
Eastern Market Postcard in a 6" x 4" photo frame
Greeting Cards in standard photo frames
Pumpkinseed Greeting Card in a 6" x 4" photo frame
Massachusetts Greeting Card in a 4" x 6" frame