Common sizes for original equipment manufacturer car speakers for vehicles sold in the United States are 3 inches, 5.25 inches, 6.5 inches, 5 by 7 inches and 6 by 9 inches. The depth of a speaker's driver and rear housing must also be considered when shopping for replacement speakers.
Online retailers such as Crutchfield and Sonic Electronix provide guides for determining which aftermarket speakers fit into each speaker location for most common vehicles. Since mounting kits are available for smaller speakers being installed in locations designed for larger sizes, do not be concerned if there are multiple sizes listed for a location in your vehicle. If you know the size of the hole in your vehicle's interior and the speaker matches it but is not listed as a fit, it could be that the speaker is too deep to fit in the vehicle's factory location. Modifications can usually be made to enable a fit, but often at considerable cost.
Crutchfield's Outfit My Car tool goes a step further by indicating whether each aftermarket product can fit with slight modification. Speakers that fit right in are color coded green, those that fit with simple modification are coded yellow, and those that don't fit without heavy modification are coded red. Both Crutchfield and Sonic Electronix offer automotive electronics for sale and ship to all 50 states and some other countries.