How small could you make this? Could you (maybe not with diy tools) make it the size of a firefly?

Much smaller. Definitely smaller than the LED itself.

https://hackaday.com/2025/03/19/worlds-smallest-blinky-now-e...

	▲	polishdude20 4 days ago \| parent [-]
		I wonder if you could do some sort of RF harvesting to get the led to blink at like 0.25 hertz

▲

bombela 4 days ago | parent | prev [-]

The size of a button cell if you have the patience to solder SMT parts, or make PCBs (at home or ordered). You can also purchase breakout board PCBs designed to adapt SMT (surface mount) chips to DIP (legged parts). You can quite easily use them to solder SMTs on it, cutting traces and bridging others with solder to make your own circuit.