Commonly used sizes are 40cm*60CM, 50cm*70MM, and some even use 60cm*90CM.
The general usage is that after each team gets the flag, it draws a mark on it with a marker pen, and everyone signs it and writes the team slogan.
Generally, you should also prepare a flagpole for each flag, which is about 1.5 meters long. PVC pipes or bamboo poles are more suitable.