I'm surprised I'm not the only one who loves shit like this. To be honest, I was expecting you guys to call me out for my nerdy antics.
The pencil sketches (or Douga, as they are known) are hand drawn, with different animators taking ownership of different scenes or sections of the movie, based on their personal strengths. The sketches are then traced by hand onto the cels/acetates, before their reverse is hand painted based on the colour coding that you can see in red ink on large Tetsuo kicking sketch (behind the cel). Quite a lot of work, considering the process is repeated 24 times for every second of the film. In some cases, up to nine cel layers are onscreen at any point in time, plus the background painting.
The Tetsuo falling/hallucinating cel is standard size (Although multi-layered), which is around 13.75 inches x 9 inches. The bulk of Akira cels and cels in general are that size. As you can see in the group shot, the others are larger, due to the level of detail required or need for panning, as in the case of the cel featuring Kaneda's bike.