The AirValent CO2 Monitor is the smallest carbon dioxide monitor you can purchase. Despite its small size, it houses a battery lasting 5-30 days (depending on your reading interval) and an accurate photoacoustic sensor. However, the sensor does have a few caveats that you can read more about here:
breathesafeair.com/airvalent-...
Apologies for the bad audio in this one. I spent hours trying to fix it to no avail. I hope it turned out okay all the same!