Various data reflect size:
The main 3 factors are:
1: Trophies won
2: Seasons in the top flight
3: Historical average attendance
But here's also
4: Highest attendance
5: Level of away support
6: Finances, record transfer paid out/ record transfer received
7: Stadium size
8: Number of social media followers
9: Reputation
10: Matches played in Europe
11: Potential- size of a city and surrounding population
and there's usually a weighting towards current day....so any big club stats from pre war shouldn't be as significant.
The Premier League currently holds 20 clubs and when ever there's a study to decide based on size which 20 biggest clubs should be in the top flight
We always come either 18th or 19th, based on the factual measurements.