Review on 🔌 Smallest Arduino Microcontroller Seeeduino XIAO - SAMD21 Based, Enriched Interfaces, Fully Compatible with Arduino IDE, Ideal for Projects Requiring Arduino Micro, 1pc by Gerardo Cibrian

I used one of these on a recent project and wanted the smallest size I could get. I don't know if it's the smallest or not but it was quite small. I was able to get it working in about 15 minutes: downloaded the latest Arduino, downloaded the drivers from the provided URL, plugged in the USB and it worked. I soldered and installed the pins upside down to minimize the vertical space needed for the pins. In this configuration it is still assembled on a solderless breadboard for prototyping and testing.