The STM32L422RBI6: That flash footprint leaves room for a full HAL, RTOS, and application stack — the real constraint is the 40 KB SRAM, which determines the maximum buffer size and task count before the linker starts complaining.
64-UFBGA 5x5 mm — board-fit decisions
The exposed paddle under the package is the primary thermal path; the datasheet's recommended copper area on the top layer sets the junction-to-ambient resistance. With 52 I/O available, you get enough pins for a parallel LCD, external memory bus, and a handful of sensors without multiplexing. The Quad SPI interface on the connectivity block (I²C, SPI, UART/USART, USB, Quad SPI) allows a fast external flash or RAM expansion without sacrificing GPIO.