Smallest launchable nuke I've seen is a two-man 102mm recoilless rifle version.
Presumably the Davy Crockett from the 60's.
I guess if you had powered armour, you'd be strong enough to tote it under your arm.
The 60's also gave us the Aeropack.
The trouble is that the more technology you have, the more likely it is to break down at a critical moment, or to screw up and kill the operator.