This is my approach to probably the smallest and lightest STM32F3 based flight controller with an integrated FrSky compatible receiver so far — tinyFISH FC.
Even though this flight controller is tiny it includes a lot of features you will rarely find on bigger FCs yet. And the best part: it weights only 1.7grams.
16x16mm hole to hole spacing (20x20mm outer dimension)
integrated FrSky compatible SBUS receiver (basically an integrated uSKY rx)
MPU-6000 SPI gyro
integrated Blackbox dataflash
integrated voltage- and current-sensor
rgb led output
(optional) add on PCB for quad brushed support
And the best: This is completely open source and open hardware! This FC will run the opensource flight control software betaflight, the opensource receiver firmware Opensky, and all hardware design files are open source as well!