* Check your owner's manual: This is the best place to start. It might specify the speaker sizes.
* Remove a door panel: Carefully removing a door panel (there are many online guides showing how to do this safely for your specific model) will allow you to directly see the speaker and its mounting size.
* Consult a parts website: Websites like AutoZone, Advance Auto Parts, or others selling car parts will let you search by your vehicle's year, make, and model. They typically list the factory speaker sizes.
Without more information about your specific 2000 Chevrolet pickup (e.g., Silverado, S-10, model, trim level), a precise answer is impossible.