Power consumption should be comparable to the blue pill with leds and regulator removed. Never got around measuring it myself but here are some results for the bluepill: https://www.stm32duino.com/viewtopic.php?f=3&t=658&start=40 , stating 13µA in STOP mode. Also note that last time I checked, low power was not supported in MySensors for these MCU's so you would need to add it in yourself.