I get asked quite often what is the smallest GPS tracker on the market.
Unfortunately there is a trade off between GPS tracker size and the length of time it will work between charges. Inside a GPS tracker you have a GSM chip with SIM card and associated circuitry, A GPS chip with associated circuitry and a battery with associated circuitry.
The battery is the largest component in the tracker by far taking up on average 80% of tracker space.
The smallest readily available tracker on the market is the size of a wrist watch. A tracker this size will work for a maximum of 12 hours between charges. If you want a tracker to last longer it needs to be physically larger to accommodate the battery power needed.
There are other trackers available that are even smaller than a wrist watch but to save power they do not have a GPS chip installed and instead use triangulation between transmitters which is only accurate down to +/- 1km