I've now checked also images from the 800D and 77D models:
DNG-Converter converts to 6024x4022, and DCRAW reports 6024x4020.
And I found that with both models the additional top rows of the DNGs are black!
So the correct size seems to be 6024x4020.
Since there is no chance to have a common and correct size, I think it might be the best to not change anything.
I've now checked also images from the 800D and 77D models:
DNG-Converter converts to 6024x4022, and DCRAW reports 6024x4020.
And I found that with both models the additional top rows of the DNGs are black!
So the correct size seems to be 6024x4020.
Since there is no chance to have a common and correct size, I think it might be the best to not change anything.