Tinyduino Is an Ultra-Compact Arduino-Compatible Development Board
Jim Heaney designed a board called Tinyduino that is just 80 percent of the size of the Nano.
Arduino boards are so popular that the name is almost synonymous with "microcontroller development board." But there are several other development board brands on the market today and they all have their own advantages and disadvantages. One of those will probably suit your needs, but sometimes you want something tailored to a specific project. Development boards are really just breakout boards for microcontrollers and so anyone with PCB design skills can create their own. Jim Heaney did exactly that to make Tinyduino, which is an ultra-compact Arduino-compatible board.
Most people select the Arduino Nano when they're looking for a small development board, but the Tinyduino is even smaller. Its footprint is about 80 percent of that of a Nano and it has one other big advantage: all of the components are on the top side of the board. That means that the Tinyduino is easy to mount, because you don't have to worry about clearance or shorting components on the underside. Tinyduino is also very affordable to build, particularly if the PCBs and components are purchased in bulk.
At the heart of the Tinyduino is a Microchip ATtiny806 microcontroller. That is less powerful than the Nano's ATmega328 microcontroller, but it is also more affordable and uses less power. The only other IC of note on the board is a CH340, which handles the USB-to-serial connection for uploading code and serial communications. Despite the small size and relatively underpowered microcontroller, Tinyduino has a lot to offer. It has 18 GPIO pins, including 12 analog inputs. The clock speed is up to 20MHz. There is an onboard LED attached to pin 0. The microcontroller supports I2C, SPI, and UART, and their pins are reassignable. And, most importantly, it is completely compatible with the Arduino IDE with the MegaTinyCore package.
Heaney said that he plans to upload the Tinyduino schematics to GitHub, so hopefully you will be able to make your own board soon.