Uruks are the more taller enemies in armour, they're slightly taller than humans, aragorn mostly. They're never the small ones, if you do not have this trophy by the end of the game like I didn't, then do what I did and load up the final chapter "The Black Gate".
(Note: completing the game lets you chapter select.)
The Black Gate has lots of goblins, orcs, Uruks and trolls, so you will get a big amount of kills on all races, this level is also great for the total kill trophy