The only 'big club' rule is common sense. If you think smaller clubs currently in the PL are bigger than us (and presumably Leeds, Sunderland etc) then you're on your own, as the replies to your post demonstrate.
Regardless - Blades are the biggest European club never to play in a European competition.
There has to be a criteria regards what facts are considered regards determining a big club
....agree there should be a level of common sense but everyone has different common sense....so it's better to aim to be more quantifiable.
The most common factors considered are
1: Historical average attendance
2: Highest ever attendance
3: Even size/ capacity of a stadium is an indicator of size of club.
Often thought if a club was a build a brand new stadium, then the capacity would be an indicator of fanbase.
4: Fanbase, this one can be more subjective, is it based in shirt/ merchandise sales, website activity, social media followers.
For example Palace fans might say they are bigger than us because they have a bigger fanbase, who knows?
Away support can be an indicator of fanbase because some clubs have fans spread around the UK, so always have high away numbers, no matter what.
Agree with Revolution in that...surely current and more recent data should carry an extra weighting compared to stuff that happened pre war.
The more recent....the bigger the weighting. Our record since the Premier League started in 1992 is pretty poor.....both Sheffield clubs have underperformed..in fact Yorkshire as a whole has been terrible in recent years.
Other big club factors are
5: Honours won, league positions more important than cups
6: Seasons in the top flgiht
7: Wealth
8: Potential, this is another subjective one, usually based on local population, hence why clubs like Bristol City, Plymouth, Cardiff are sometimes referred to
as "big clubs" when the historical data suggests they are all small clubs. Virtually all London clubs, could claim to be big due to their potential.
Minor big club factors
9: Seasons played in Europe/ reputation
10: Largest transfer paid/ latest transfer received
11: Number of internationals etc etc.