http://www.non-league.de/phpkit/I recently came across this website which is devoted to non league football in Germany, with lots of stadium pictures. They certainly have some huge grounds in the non-league with some great terracing, much of it rather decrepit.
A lot of the grounds seem to be oval shaped with terracing virtually all the way round, and some of the capacities are huge. Given this I was wondering if Dortmund did actually have the biggest terrace. If you follow the links to Stadien (on the left side), then click on NRW, the second ground down is the 36,000 capacity Jahnstadion at TSV Marl-Huis, it looks to be one terrace all round apart from the small stand, which would make about a 34k terrace. Bigger than Dortmund if it is!