To know for sure, you need to:
* Check your owner's manual: This is the most reliable source.
* Physically remove a speaker: This will allow you to measure the speaker directly.
* Consult a parts catalog: Using the year, make, model, and trim level of your truck in an online parts catalog (like AutoZone, Advance Auto Parts, or Napa Auto Parts) will usually show you the correct speaker sizes.
Without more specific information about your 1997 Chevy truck, I cannot give you the precise speaker sizes.