Dortmunds is i think around 65k for europe and 80k for Bundesliga - but think about it, they have one other club within 80 miles of them, Schalke, there is a reason they can get 80k crowds for games, they also have the entire Dortmund region as the only top flight club.
Meanwhile Everton, second club support wise in the city (certainly from a country wide perspective), within 55 miles, they have Liverpool, Man City, Man Utd, Stoke, Huddersfield and Burnley currently, 'historically) you also have all the Yorskire teams such as Sheff Weds, Utd, Leeds, Blackburn and numerous other sides which have decent support due to their clubs past.
Looking at the German model is a bad way to try to view what we can do, and looking at German fans as a model is wrong, because simply put no ground regardless of it's size can come close to matching the German games atmosphere and they won't any-time soon, the German model of ownership actually connects the fans to the club and its why RB Leipzig are hated, the premiership model is to seperate fans from the game, no 70k ground will change that it will just make it easier.
Juve new stadium - 41-42k - there old one i think had close to 70k
Roma new stadium - desinged by Meiss is gonna be 55k - they are moving from the Olympic which has about 70-75k atm
Portos - 50k (their prior one was 55k)