Well, they're these, three inches by three inches, to be exact. I'm not sure where or how they originated, but likely as a variation on the popular Artist Trading Card (ATC) which is baseball card-size rectangular.
These pictured above are for an Itty Bitty Swap and project challenge by ARTchix Studio http://artchixstudio.com and our Yahoo group. The requirement was to use faux postage stamps from ARTchix, and since two of my faux pet post stamps were just published by ARTchix, of course I jumped right on this. I also used some of my "real" cat postage from Zazzle.com, reasoning that I would spend more than 41 cents on other supplies or clip art anyway. These are a spoof on high-end designer purse ads for those non-fahionistas reading this.
ART chick Susan Burgess, aka Sanna, gives a great tutorial on itty bitty cards here: http://www.artchixstudio.com/create/classroom/class_ittybittys.htm. Very cool; she got me going!