What do the miniature sizes mean?
Dungeons and Dragons Miniatures may have the following sizes:
Tiny—a base smaller than 1 inch
Small—a base smaller than 1 inch
Medium—a 1 inch base
Large—a 2 inch base (meaning it takes up 4 squares on the typical 1 inch scale battlemap)
Huge—a 3 inch base (meaning it takes up 9 squares on the typical 1 inch scale battlemap)
Gargantuan—a 4 inch base (meaning it takes up 16 squares on the typical 1 inch scale battlemap)
While the base sizes are fixed the actual size of creatures on that base can vary quite a lot, especially in the bigger size groups.
It is worth noting that WizKids' base sizes do not always match the stats presented in game. Often this is for practical reasons - a sculpt might not be stable on a tiny base for example. Or the packaging might not accommodate an appropriate base size. Sometimes it is simply a mistake.