One of my all time favorite sets from the 1990’s has got to be Topps Micro. They were only issued as sets and can commonly be found in Target 100 card packs. They are absolutely worthless (as this auction on eBay can attest to) but damn were they not the coolest cards. Their size was about the size of a quarter and I’m guessing they would run 4 per regular cards if laid end to end.
Imagine some of your famous cards as micros, like a Stephen Strasburg rookie card. Or how about a draft mini-set containing Bryce Harper. In 1975 Topps did it as an experiment and not only were the 1975 design one of my favorite designs (you know back when they really cared about designing cards). The 1975 minis are one of the most collected odd ball cards of all time.