The literal translation of my sentence is "You look like a German". This is the most accurate translation of your sentence.
As for the word "si", it simply means "like", and in Albanian there are some specific cases where it can be omitted, especially when the verb used is "to look like/seem/appear" (though there are cases where "si" can't be omitted). Ex.: "Po dukesh mir" ("You look good") or "Po dukesh fëmij" ("You look like a child").
Therefore, "Po dukesh gjerman" is correct too.