I'm not too sure about short keepers either but 189 cm is a decent height for a keeper.
Pepe Reina is 188 cm tall.
189 cm in feet is 6ft 2in
Karius would be one of the shortest keepers in the premiership.
Only Schmeichel and Speroni are shorter.
6ft 2.4 to be precise. So he's around the same height as Adrian, Vorm, his almost-namesake Lloris, Fabianski etc - very normal GK height, not fair to say he's "short," just not one of the super-tall ones.