Are there any rocket engines that are small and light enough to be held in one or both hands? Gas, liquid and hybrid designs are fine but solid motors are excluded (with the exception of solids that are some way exceptional from the rest). Along with engines, small thrusters are also good.
Colloid microthrusters are probably the smallest.
If you want liquid, I'd say RD-4D RCS thruster used on Apollo spacecraft, at 8lb weight would be a reasonable example.
Chamber Pressure: 6.93 bar.
Area Ratio: 164.
Thrust to Weight Ratio: 13.7.
Oxidizer to Fuel Ratio: 1.65.
Coefficient of Thrust vacuum: 1.89.
Unfuelled mass: 3.63 kg (8.00 lb).
Height: 0.55 m (1.82 ft).
Diameter: 0.28 m (0.92 ft).
Thrust: 490 N (110 lbf).
Specific impulse: 312 s.
The MR-106E rocket engine is most likely the smallest liquid rocket engine. It uses hydrazine monopropellant. It could easily be held with one hand. It has a length of 18 cm, and weighs about 1 pound (about 500 grams). It was used on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite.
Rocketlab's sea level Rutherford engine is probably near the upper limit. It masses 35kg, and estimating from the 25cm nozzle diameter is about 75cm high. Despite it's awkward shape, that would still be single hand lift-able by a person of average/somewhat above average strength.
(Originally from Rocketlab)
Of course there are the RCS (Reaction Control System) engines used in every manned space ship as well as in larger satellites and space probes like Voyager, Cassini-Huygens and New Horizon. Small cubesats don't use them.
See the red markings in the drawing of the Apollo CM and the LM image of Apollo 11.
We can't mention the Hypercurie and ignore the Curie (on the right):[A sneak peek at a hypercurie nozzle (left) vs. std Curie. Hypercurie is our latest spacecraft engine, it’s Electric pumped, on orbit storable, non toxic and It’s perfect to get payloads to the moon and..........Venus!
–Peter Beck on Twitter]
You see the Hypercurie nozzle for scale. The Curie is tiny.
Since Dan Is Fiddling By Firelight already mentioned Rocket Lab's Rutherford engine, we should also add the Hypercurie:[The new HyperCurie engine for deep space and planetary missions on Photon. Banana for scale, apparently bananas are now recognized units of measure?
–Peter Beck on Twitter]
I don't know the mass of this engine, but unless it is made from depleted uranium, it can't be that heavy. Also, it's made for spaceflight …