The size of a speaker can be expressed in different ways that depend on the purpose of the measurement. A single speaker can be one size for installation purposes, another size for the purchase of a repair kit, and yet another size for expressing potential power.
For installation, speakers should be measured straight across their widest point. This is the maximum diameter and the minimum space needed in the speaker box to accommodate them. Repair kits should be bought with a measurement of the interior diameter of the speaker, as this is the section that needs work. Finally, speaker diameter is usually expressed by professionals as the external diameter of the speaker minus the mounting lugs.