Surely -NOT- the smallest, but the smaller military apparel I can think of (my knowledge of military radio-communication equipments is limited at best).
The tactical beacon or TACBE as it is often known has been used by British and American armed forces for many years. Its primary role is to contact planes or helicopters which may be overhead or nearby. The system's main use is a distress signal, sending out a constant danger message to AWACS planes. However, it can also be used as a short range communications device with local aircraft. Using it indicates that one is in danger and needs help. Its most famous use is by the SAS team Bravo Two Zero who, after becoming lost in Iraq (1991) and suffering casualties, tried to call in helicopter EVAC or aerial support to no avail.
Some pretty neat tactical radio-communications apparel can be seen HERE
, that's the InvisioBone Mic from TEA Inc.
"It is criminal to teach a man not to defend himself, when he is the constant victim of brutal attacks. It is legal and lawful to own a shotgun or a rifle. We believe in obeying the law." -- Malcolm X
"We (atheists) act in good conscience because we believe in moral principles, not because we expect a reward in Heaven." -- Margherita Hack