We are not as big as Newcastle, Villa, Leeds, Everton and the so called big 6.
IMHO we are in the same bracket as Sheff Wed, Sheff U, Forest, Derby, Leicester, West Brom, Wolves, Coventry, Norwich, Southampton, Stoke, Sunderland. You can make a case why we are bigger than them, or smaller than them; depends upon the criteria you value most and timeframes you respect most.
Bristol City and Cardiff have the population to crack it, but never seem to sustain it.
Ipswich and Pompey have just disappeared and may never return.
Blackburn is the one I can't take - much smaller place than Teesside/our catchment and I class them like Burnley, Preston, Bolton, Huddersfield as clubs with proud old histories, but surrounded by big clubs and unable to grow. Even when they do well, they get poor crowds and poor level of interest.
I know Palace, Fulham, Watford, Reading, Brighton, Bournemouth are from more affluent areas, but I just see them as newbies, where their fans are new in or just don't seem to care as much.
It doesn't matter much anyway really as biggest matters a lot less than where we all actually are.
(Stoke is 6 towns by the way)