* Check the speakers themselves: Remove one of the speakers (carefully!) and look for markings on the speaker itself indicating the size (e.g., 4", 5.25", 6x9").
* Consult a wiring diagram: A wiring diagram specific to your truck's year, model, and trim level *might* show speaker sizes, though this information isn't always included. These diagrams can sometimes be found online through forums dedicated to classic Chevy trucks or through a repair manual.
* Look for speaker mounting holes: Measure the diameter of the mounting holes in the dash and rear panels where the speakers are installed. This will give you a good indication of the speaker size.
Without knowing the specific sound system option in your truck, it's impossible to give a definitive answer. It's quite likely the dash speakers are smaller (4" or 5.25") and the rear speakers may be larger (5.25", 6x8", or 6x9"). But you need to physically check your truck to be sure.
