Hi @Slashbrackets, the smallest COTS GPS tracker device that I use is the DigitalMatter Yabby.
The manufacturers publish a very good Javascript decoder that works reliably on TTN. They are easy to configure over USB using a downloadable Windows PC utility. By default they uplink based on motion. I also configure some for per 15-mins uplinking.
Most of the volume of the device is taken up with 3x AAA batteries, not the electronics which are very small.
I guess that you could find smaller devices at the cost of battery life.