Really do come in all shapes and sizes, and in a lot of cases they can be used for bluegills as well.
From 10 to 14 foot cane poles, even though glass has replaced the bamboo, to 4 and half foot ultra lights. Here is a quick breakdown of their uses.
Cane poles are for being stealthy from a distance and dropping down right on top of the fish. Just watch out for the tree branches when lifting up! These are also great for kids starting out.
Long rods with spinning reels can be used for that too, but also for drifting or trolling for crappie. Used with plastics or live bait, this is an effective method catching on in the North, but has been a standard practice down South.
Six to 7 foot ultra lights are mainly used for rigging with a bobber. Cast out, and wait for the hits.
Light action rods, in the 6 to 7 foot range are for casting into cover. Rushes, weeds and cattails for some examples, but also for covering water around docks.
The smaller rods are for casting in to tight areas. Tighter the spot, the shorter the rods is a good general rule. For covering open water, a six or 6′6″ rod will do the trick.