There are a lot of options. It really depends on what you want. How small, how bright? How many hours? Etc?

The Cygolite Dice Lights are small and compact.


The headlight will give an hour or so at high, I think. A bit longer at low beam.

Light & Motion Makes several lights about the size of a AA Penlight. They put out a respectable amount of light. But, over time I wore out the USB port making it into an expensive paperweight. I think the rain cover over the USB port also went bad.



There are quite a few other penlight style bike lights that you can look at.

Some of the China-Direct CREE lights with single bulbs and exernal battery packsare smaller than they'd otherwise look in the ads. Of course, you'd still need to find a place to put the battery pack.



I think this one is USB cable powered, but there should be small 8.4V ones too.