I built all the micropython stm32 targets from the git master and the ESPRUINO_PICO was the smallest but even that exceeds 256KB of text:
text data bss dec hex filename
263240 28 44352 307620 4b1a4 build-ESPRUINO_PICO/firmware.elf
Espruino Pico website says the device has 384KB of flash and this matches the
stm32f401xd.ld linker script.
MICROPY_HW_ENABLE_RTC reduces the size to just under 256KB.
I think the networking is already disabled in the PICO configuration:
When I used micropython on other boards they normally appeared on the USB bus as both a serial port (USB-CDC) for accessing the Python REPL and a USB mass storage device for uploading python scripts. Storing the scripts on the device obviously relies on having some unused flash sectors left over so the image would have to be shrunk further for that to work. Would micropython still be useful if it only had the interactive REPL?