Spinning reels are among the easiest fishing reels to use. As with all reels, spinning reels come in many sizes. In general, spinning reel sizes can be labled as follows; ultra-light, light, medium-light, medium, medium heavy, heavy, extra heavy.
The selection of the reel however, should be based on the target species, the area you are fishing in, and your fishing technique.
For inshore fishing for fish such as Snook, Sea-Trout, etc., a medium sized reel is adequate, however, if you are targeting larger fish such as Tarpon, you're going to want to go with a heavy or extra heavy reel.
If you are going to be fishing in areas where there is a lot of structure, or high currents, you may want to go with a reel a size up, if you want more of a challenge, you can use an even lighter rod.
Another thing to consider is what type of line you are going to fit your reel with. The new braided lines are stronger and lighter, making for a better casting line. You can hold a longer length of heavier braided line on a reel than monofilament, but you need to make sure the reel is designed for braided line.
As a generality, the following could be said regarding reel size to the line size:
Ultra-Light - 2 to 6 lb. test line; good for small fresh water fish such as bream, and small bass
Light - 4 to 8 lb. test line; good for medium sized bass, etc.
Medium-Light - 6 to 10 lb test line; good for fishing snook or sea-trout on the grass flats.
Medium - 8 to 12 lb test line; good for fishing snook or sea-trout around the mangroves, and moderate structure.
Medium-Heavy - 10 to 14 lb test line; good for fishing snook around bridges and high current areas, or for a challenge, Tarpon on the flats, Spanish Mackerel, etc.
Heavy - 20 to 25 lb test line; good for Tarpon, Spanish Mackerel, King Mackerel, Dolphin (Mahi-Mahi), Blackfin Tuna, etc.
Extra-Heavy - 25 to 40 lb test line; good for King Mackerel, Dolphin (Mahi-Mahi), Blackfin Tuna, and pretty much everything except the biggest off-shore fish (Marlin, etc.).