These traps are approximately 14" wide and have 3 flews (a flew is the entry for the crawfish). The 4th corner is for putting in bait and removing the crawfish.
The shorter traps catch a little better, because the flews are closer together. The large traps keep crawfish alive longer when oxygen is a problem. The 24, 30, and 36 size indicates the width of the wire these traps are made with. After "fluffing up" for fishing, the length of the trap is about 2" less than the wire width.
The traps come flattened out for shipping. To prepare the traps for fishing, each trap has to be pulled from each side to form the pillow shape. You can then push something like a soda bottle in the flews to open it up so the crawfish can get in.
Although we have a commercial crawfish bait, many people use scrap fish or raw chicken parts for bait.