Link to video - https://youtu.be/7J-0tpTtF-8

Magma cubes are supposed to require a 3 x 2 1/2 x 3 block space to be able to spawn. My testing did not cover the X and the Z spawning requirements, and only the Y spawn requirements.

My tests show that a magma cube will spawn in a 2 block height location. This prevents the player from being able to eliminate magma cube spawns by placing a 2 height ceiling. Also, when spawning into the 2 block height space, sometimes (not all of the times) it will spawn with the bottom of the mob being level with the bottom of the floor block (mob is clipping into the floor). This has been seen to allow the mob to then pass through solid walls to get out of clipping with the floor as he pathfinds away from the location.