Rule of thumb for the minimum space needed to squeeze through is to spread you hand wide, getting the tip of your thumb as far away as possible from the tip of your little finger. That is a hand-span. Two hand-spans wide and one tall describes the rectangular or ovoid hole you can get through. I have made it through spots slightly larger than 8" high by 16" wide.

Turns, particularly turns that bend you backward, will cause you to need more room. Vertical holes, particularly if taken head-down, are particularly hazardous. Unexplored routes can be extremely hazardous.

I know one cave there the guy who first explored it nearly died when a long downward squeeze opened abruptly into a room where the floor was better than thirty feet below and the walls were smooth. Luckily he had used a length of webbing as a belt and was able to stop himself and use the webbing to form a stirrup hung from a protrusion in the squeeze so he could hang over the precipice and reverse direction.