Is this to small

Would a 24x12x12 cage be to small for a semi adult BP

Yes. General rule of thumb is one square foot of floor space per foot of snake.

depends what u mean by semi-adult, but probably too small.

I'm curious about this size being too small, is it that the length isn't long enough or is it not tall enough. Because by the looks of all the racks I see, none of the plastic containers are over 6 inches tall, must be length then?

Balls are ground dwelling snakes and need very little in the way of height, so 6 inches is all they really need. Floor space is much more important.

most ground snakes dont need tall as much as they do long. a good rule of thumb i herd once is that a cage should be 75% of there length.