This is the overall diameter of the speaker's basket. Standard sizes are 3", 4", 5-1/4", 6-1/2", 8", 10", 12", 15", and 18". Actual measurements can vary significantly regardless of the manufacturer's size designation.
This dimension corresponds to the minimum diameter of the round opening into the inside of the cabinet into which the speaker will fit.
This dimension is the overall depth of the speaker. Note that the measurement is taken from the surface of the gasket, and not from the bottom side of the flat ring which bears the screw holes.
This dimension is the overall diameter of the permanent magnet, round on virtually all modern speakers.
This is the depth of the driver assembly, containing the front and back plates and the permanent magnet.