the rugby players have at the back of their rugby shirts, i noticed it whilst watching the Rugby World Cup. The item is near to the size of a cigarette packet. Or possibly be a bit smaller. I think the footballers have them as well.
the actual tracker can be quite small, so I would imagine they are encased in foam or rubber to prevent any damage to the player if he lands on it (and not damage the tracker either)
I clicked on this feeling just a little smug because I knew the answer -alas, others are there before me. :( Rather 'Big Brother is watching you' so that players can be instructed and better trained. We're watching as much as appears on terrestrial TV, but I much prefer Rugby League ---- final of Super League coming up shortly. If you like rugby, please watch - League is much faster and more exciting. Took a while for Mr. J2 to come around to that viewpoint, but he now agrees.