For a really tiny radio (AM band only) the vintage Standard Micronic
Ruby's are about 1/8th the size of cigarette pack.

My SR H436 outperforms all other shirt pocket transistors I've used (a
lot), and runs cheaper than those that use those modern 9volt types,
thank to Wumpus:

http://www.oldradioworld.de/srh436.htm

(Using modern hearing aid batteries, instead of expensive vintage
sizes).

That outer cardboard wrapper is completely unnecessary.

Just stack two naked (size 675) hearing aid batteries and a brass
(size 6-32) bolt/washers/nut spacer wrapped with a cardboard tube (or
duct tape instead any cardboard at all), to make the permanent spacer
wider so it won't roll around.

Ruby's look cool!