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?
Answer: Many of the sound effects use an electictronic instrument called a theremin which was invented in 1919. You may hear it used more musically in The Beach Boys song "Good Vibrations." The visual effects would have been animated composites as used in Star Wars and Ghostbusters.
Myridon