Plot hole: Krell doorways and equipment suggest they are short, wide and 2-3 times larger than a human. Yet the shuttle pod the Commander, Doctor and Morbius ride in is designed for human size beings.
Plot hole: Morbius says no physical representation of the Krell remains. Yet later he shows the Commander a screen that contains the total sum of the Krell's scientific knowledge. So didn't the Krell study their own biology?