Anyone have experience with Franchising?

I have a business I am considering franchising and wondered how others have structured their franchises.

How granular do you go? Do you split it up by individual postcodes, cities, counties, countries? Do you off the flexibility to get the franchise at different granularity and charge accordingly?

I am UK based so these terms will be different in each country. E.g. Postcode = Zipcode, County = State (although some American states are larger than the whole of the UK)

For context it requires a physical presence so geographical area is important for travelling time etc.