Bass traps: determining size and how many

I saw that someone had built some huge traps, 24 X 48. My question is, how do you determine the proper size. What is the correlation between the frequencie that you need to trap and the size of the bass trap? Will one huge trap, say 24 x 48, be as good as two smaller traps: two 12 X 48?
Short answer, no. Bigger trap = lower frequencies so two smaller ones is not the same.

Long answer, first you need to determine what the problem frequencies are and then determine how to address the problem; absorb? equalize? change modes (dimensions) of the room? etc? This can a very complex process and will require a lot of research and some test equipment on your part or the help of the pros.