The most common sizes are:
* Front doors: 6.5" (most common) or sometimes 5.25" (less common)
* Rear doors (if equipped): 6x9" (most common) or sometimes 6.5"
* Dash: Typically small, 2" to 4" depending on the location.
To find out for *your* truck:
* Check the existing speakers: The easiest way is to remove a speaker (carefully!) and look at the size printed on the back.
* Consult a parts catalog: Use the VIN number of your truck with a parts website (like AutoZone, Advance Auto Parts, or a Chevy parts site) or a manual to identify the exact speakers your model came with.
* Look in the owner's manual: If you still have it, the manual may have specifications about audio equipment.
Without knowing your specific truck's details, it's impossible to give a definitive answer.