Same size as popular RS485-TTL module for more compatibility.

As software part you can use:

OpenTherm gateway for HomeAssistant - very intelligent OT control firmware 🧠

Tasmota OpenTherm - perhaps the easiest way to connect the boiler to your smart home system 🏚

Opentherm IO library for Arduino or OpenTherm Arduino/ESP8266 Library (slave mode only) if you want to build your own firmware 🛠

Some items ship from Germany and others from China. Please check.