I never knew what an ATC was until I saw swaps on Swap-Bot for them. I Googled ATC's to find directions on how to make them. I have made a few since then. We have a swap going on now for fabric ATC's. Here are two that I made. One I mailed off, the other is a surprise for someone else.
They measure 3 1/2 inches x 2 1/2 inches.(The same size as baseball cards.) I left the lace open on one end and inserted a button before sewing it closed.