• >pio init --board bluepill_f103c8 --project-option="framework=arduino"
• then update content of platformio.ini

Content of the platformio.ini

[env:bluepill_f103c8]
platform = ststm32
board = bluepill_f103c8
framework = arduino

The main.cpp

#include <Arduino.h>

void setup()
{
  pinMode(PC13, OUTPUT);
}
void loop()
{
  digitalWrite(PC13, HIGH);
  delay(200);
  digitalWrite(PC13, LOW);
  delay(1000);
}

The compiler is the same "arm-none-eabi-g++" and the result is

arm-none-eabi-size -B -d .pioenvs\bluepill_f103c8\firmware.elf
text	   data	    bss	    dec	    hex	filename
6284	   1920	    312	   8516	   2144	.pioenvs\bluepill_f103c8\firmware.elf

So the Flashed code size is : 8 KB