5x8 Index Cards
Scale changes a lot.
For example, you can get a much more complete thought on a big card.
One way I was tagging cards as groups was with the same coin envelopes I use to tag piles of 3x5 cards. Here's some photos of that.
Each envelope has an unregistered thinglink (badly formed; I was doing it by memory and I got the format wrong). I need to work out some way to print registered thinglinks automatically, like with an inkjet printer or a dymo labelprinter.