fwiw this was a study on how many supporters each team has. Just supporters and nothing to do with actual members or revenue etc. Hard to say how accurate it is, but it is kind of interesting.
http://www.roymorgan.com/findings/7345-roy-morgan-afl-supporter-ladder-2017-201709210118
Problem is numbers fluctuate too much eg. from 2016 to 2017 Essendon grew over 100 k fans and Collingwood lost over 100 k.
Plus they only survey 14,000 people over the age of 14. Not a big enough survey imo to get a true estimate of who is the biggest club