Artist trading cards are always 2 1/2 inches by 3 1/2 inches. They are the size of a 'baseball' card and fit into the plastic wallets that collectors acquire.
The original atc's link is here http://www.artist-trading-cards.ch/
Loads of information here http://www.cedarseed.com/air/atc.html
see some photos of atc's here http://www.flickr.com/groups/95067770@N00/
and some textile based ones here http://www.quiltart.com/apc.html
They can be made from anything with embellishments galore, the only other rule apart from the size is that they should be traded and not sold/bought. There are quite a few groups on the internet who trade these cards.
:) Wendy
p.s. I have a few, if you want me to bring them to a meeting, please let me know
