Usually shepherds are quite long and thick, I dont' think they can be thin excepting some specimen of GSD. Anyway, I'm agree with mastiffs, they are hot, but not with Dobbies: they are a bit aggressive and not really well-endowed, not to mention how difficult to train they are. You should check a Rottie ( they have big cocks and easy to arouse ) and Weimaraners ( in person and in videos, I am still amazed at how sexual they are and how nice and long their cocks are )
German shepherds and Weimaraners have the most perfect cocks I've ever seen.