Inspired by Ted Scannell's 4mm scale work, I've been experimenting with Nigel Lawton micro motors.
The motors are 6mm diameter by 10mm long, and nominally rated at 6v. By putting a pair in series driving the mechanism, this doubles the power and neatly gets them to 12v. Nigel Lawton also supplies belts and pulleys, which can give an easy 3.6:1 gear reduction.
I calculated that a block of metal to carry two motors and a layshaft would be smaller than a typical Tenshodo/Mashima small motor. And the shaft would be at a lower height, suitable for running onto a worm/wheel directly to a driving axle.
The total box volume is 17 x 13.7 x 10. But that includes the pulleys and the layshaft.
The wider upper part is 13.7 x 10 x 7.3mm thick, and could be thinned further in height and width. The layshaft protrudes a further 2.2mm below the main block and is 4mm wide.
( Yes, the drive belts are too slack and I need smaller ones. My mistake on the original calculations and new ones are on order ).
Update following comments:
The motors are friction fit into 6mm diameter holes through the block.
The layshaft is in plain brass bearings; they are DIY ones, 3mm O/D by 1mm thick, but I think Association parts would fit. The hole through the block is 2mm O/D, so that a shaft only touches at the ends when supported. I may substitute 3mm O/D by 1mm I/D ballraces later (and then sleeve a 1mm shaft up to 1.5 for the pulleys).