My rule of thumb is that a weapon is half as big as a character of the same size. For instance, a Medium-size human can be 6' tall, but a Medium-size longsword or rapier is closer to 3'. Obviously there are exceptions, but this is simple enough for most purposes.
Using that rule of thumb, weapon sizes would be:
Tiny weapon: 6" to 1'
Small weapon: 1' to 2'
Medium weapon: 2' to 4'
Large weapon: 4' to 8'
Huge weapon: 8' to 16'
(For a weapon near the border between two categories, consider its weight. A heavy mace is about 2' long, but since it's heavy, it counts as Medium instead of Small.)
