The kicker is not getting something that can receive and process GPS data. The kicker is getting something that can re-transmit that data to somewhere else at a useful distance.

From a technological point of view, the obvious minimum size for such a unit would be the minimum size of a cell phone. I would suspect that the real size bottleneck would be the battery.

On the other hand, people routinely have pacemakers implanted with power packs the size of a cell phone.

Again, it wouldn't take much for terrorists to defeat such a system if they knew about it. Easy solution, if they are holding someone for ransom and don't want to damage them, is enclose them in a Faraday cage of some kind. For cell phone signals, this isn't hard. Heck, the number of ordinary buildings I go into where I lose my signal is legion.