* Check the owner's manual: If you have the original owner's manual, it might specify the speaker sizes.
* Inspect the speaker locations: Remove the speaker grilles (if present) to physically measure the speakers.
* Consult a parts catalog: Use a parts catalog specific to a 1984 Chevy Silverado (like those found online or at auto parts stores) using the truck's VIN number for the most accurate information.
Without knowing the specific trim level and options of your 1984 Silverado, it's impossible to give a definitive answer.