How big should a doormat be?
The simple answer is as large as possible.
Ideally you want a mat big enough that people can enter the house, and close the door behind them whilst still standing on the mat.
This means that for those that don’t bother to stop and wipe their feet, and pets and children, there is maximum chance that both feet will have made contact with the mat on the way in.
A large mat has the added benefit of being an area where people can stand to their remove shoes and will be weighty enough not to move around.