How big was the white prison in THX1138?

The implication, at least in the novelisation, is that the white prison is large, but not astonishingly so. When he finally reaches the door, THX realises that his perception of the prison was being skewed in some way and the sides and edges deliberately hidden from him.

The whole blank background of nothingness seemed to change as a camera changes focus, bringing objects that were blurred into invisibility suddenly into clear, sharp view.

There was a door, flanked on both sides by flashing varicolored lights! And it was set into a metal bulkhead, with steel ribs protruding from it and rivets in the ribs. THX put out a hand to feel its reality.

“What… what… how can it be?” He heard SEN breathless behind him.

“They must have done something to the way we see,” THX said uncertainly. “They did something to our eyes…”

“Or maybe the food cubes were drugged,” SEN suggested.

“Or hypnosis.”