We sell the Thermocron temperature logger which, I think, is very small. Most of the time people don’t want a logger that small and we sell it with a plastic fob (holder) to make it slightly larger and easier to handle and hang.
But every now and then we have someone who wants one that is as small as possible.
We have a prototype here which is 8mm x 10mm (without battery). It is also a motion logger. And it has memory for hundreds of thousands of readings.
So it probably isn’t an everyday type logger, but we suspect that it will make some researchers start to drool.