> the size of a football field is variable

Sure, maybe the local, community field you and me play on, but not the ones where "real" matches are played. They have a standard size.

FIFA for example have the dimensions in their first law: http://www.fifa.com/mm/Document/FootballDevelopment/Refereei...

Probably they mean a standard football field.

It lists a minimum and a maximum on page 34 and it’s a fair range. Am I missing something? If I google the home pitches of a few famous clubs, they all have different sizes too.