On the computer you can create your artist trading cards in graphics software or a page layout program or use both. As with other typical desktop publishing projects, it's common to do complex graphics in a dedicated program then import them into a page layout program to arrange various elements including text.
You can also use page layout software to try out different artistic arrangements even if you plan to use embellishments such as rubber stamps, fabric scraps, die cuts, or other non-computer elements on the card after printing.
As with any desktop publishing project, you can use grids and guidelines to help you in the composition of your ATC even if much of the work will ultimately be done off the computer.
As previously stated, 2.5 inches x 3.5 inches (6cm x 9 cm or 64mm x 89mm) is the standard size for artist trading cards. You can work at larger sizes and reduce your artwork to fit, or start with those dimensions to insure your designs will look good at the reduced size.
For artists that prefer to work at very large sizes, with lots of detail, the small size of the ATCs may seem too limiting. An alternative to reducing a larger piece of digital artwork is to cut it into ATC-sized pieces and create a set of cards. This would allow you to focus on key parts of the design. The cards could be traded as a set, a bit like a puzzle to be assembled, or you could trade them individually.
The artwork in the illustration — "Girl Under Glass" — is being resized and printed from a graphics program called Paint.NET.