Not exactly answering your question, but I'm pretty sure Geocities started it. See the left sidebar in this wayback grab from 1996: https://web.archive.org/we

Not exactly answering your question, but I'm pretty sure Geocities started it. See the left sidebar in this wayback grab from 1996: https://web.archive.org/web/19961022173245/http://www.geocities.com/ https://web.archive.org/web/19961022173245/http://www.geocit... So people would use those as a starting point and customize them. Why geocities picked those dimensions? Not sure. They are in a <td> with a width of 120.