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)
STM32F3 CPU
MPU-6000 SPI gyro
supports DSHOT
integrated Blackbox dataflash
integrated voltage- and current-sensor
buzzer output
rgb led output
open hardware
(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!