How do you define it?
Attendances? (Historic as well as current.)
Followers on social media?
Wealth of club?
Pots won? (Does recency matter? Do very old ones won when women wore bustles still matter?)
Column inches in the Daily Mirror?
Famous players over decades?
Success in Europe?
History? I mean real history, not just selected highlights.
Sales of replica shirts?
Number of noodle sponsors in Thailand?
It all seems a bit vague to me. And at the end of the day, so what? If you follow a 'big club' does that make you a better person in some way? Does it mean 'your' club deserves to win things in perpetuity?
It's all a bit similar to people getting a hard-on because the British Empire used to be massive. Even though they had nowt to do with it, and were just born here.