Based on that high-quality image, Fassbender occupies 68.86 % of the vertical extent of the doorway in which he is stood. Since he is 1.83 m tall, the doorway is 1.83/0.6886 = 2.66 m high.
This is 8' 7.6", and places an upper limit on the height of the alien beings, unless they walk along stooped over....
Also, is it just my imagination or do some of the other pyramid chambers have objects in them? I can see the central chamber has the giant head, but there's another to the right that looks like it has a big mouse inside it.
I note also that the storm appears to be ongoing outside the windows, yet there are markers for Fifield and Milburn still inside the pyramid....