I just tested the newly assembled "Mini RS485 Node" based on the smallish STM8S001J3M3 µC. The board has just 1/3 of a square-inch (and a good deal of that are connectors).

The board has components on both side, and this time I also populated the DS1621S thermometer chip.

Here is the "board" in profile next to my new CH340E RS485 dongle:

The dongle provides 5V power from USB - since the MODBUS "board" has a 3.3V regulator bus powered systems measurement systems are possible.

That's certainly not cost efficient for any series production item but I wanted to know what's the smallest stuff I can handle. To be honest, the MSOP10 chip was a bit of a challenge, more so than soldering a STM8S207RBT6C in a LQFP-64 with 0.5mm pitch (ST makes the gaps wider than the pins - 0.22mm to 0.28mm - the CH340E pins appear to be a bit wider).