Customizable stadium capacities
Minor new feature in version 2018.04.08.66: you can now set the stadium capacity for each team. To do this, enable God Mode (Tools menu) and then go to Tools > Edit Team Info.
This will probably do weird things to team finances if you change it. You probably shouldn't edit it unless you're making a custom league file.
For people making custom league files, see the teams customization documentation for more info.