Raid: bigger loot window?
 

Question answered

Edit:
Now I found out, how to do it:

Is it possible to change the size of the Loot Roll Zone window - yes
Playing Draigoch (or other instances with masterloot), the loot window is too small to see all the items of the 3 chests at once.
Changed the ID_UISkin_GamePlay_ItemRollBox.xml

Resolution: 1680 x 1050