It's no problem building a pocket-sized transceiver - I've seen commercial multi-band HF SSB HTs.
With some work you can even fit a battery inside. But the problem is the antenna efficiency: the
smaller you make the antenna, the higher power you need to run for the other person to hear you,
so the larger battery you need.
If you want to talk from one room to another, perhaps over a few hundred yards, you probably
can manage with a small loop antenna (perhaps even a ferrite loopstick to save space, again
at the cost of efficiency.) If you need better range with a small antenna, move up to the VHF
range where full-sized antennas are shorter: a 54" telescoping whip on the back of a radio
works far better at 6m than it does for 80m (though there may not be many stations to work.)
If you have a friend who you want to work, you can make a pair of sets and talk to each other.
When I was a teenager I built a single transistor transmitter that probably ran as much power as
the Pixie and mounted it on my bicycle along with a CB whip antenna - I think I was heard 1/4 mile
But the best thing to do is to build a Pixie (or some other small transmitter/transceiver) and try it
out to see how much range you can get. My suggestion would be to use a simple transmitter likethis one
(which is one of my favorites) and try it out while listening on a standard receiver. Load it
into various sorts of small antennas and see what sort of range you get. Try various loop antennas
(you can connect them directly to the collector of the output stage if they are sufficiently selective
and have a DC path through them. You may be able to rewind the loading coil on a mag-mount CB
antenna, then replace the top stinger with a telescoping whip. Measure the relative signal levels
with a wire laying on the floor vs. wrapped around the box. You may find a combination that works
for your needs, or you may come up with some other creative solution. Once you've found an antenna
that works there are various transceiver circuits that you can build to go with it - consider it a modular
project, where you can keep improving different pieces. Perhaps you can design a rig that operates
on 440 MHz that is linked to a home HF station instead, which would allow you to key 1000 watts on
40m with a full-sized antenna from a tiny box in your pocket.
But the first step is to build something simple and start experimenting. You never know where it will
take you from there.