* Check your owner's manual: This is the best source for this information.
* Remove a door panel: This is the most reliable way to find out for sure. Carefully remove a door panel (plenty of online guides show you how) and measure the speaker mounting hole. This will give you the exact size you need.
* Consult an online parts database: Sites like AutoZone, Advance Auto Parts, or RockAuto allow you to search by year, make, and model. Enter your truck's specific details to see what speakers are listed as compatible. However, even these databases may not be perfectly accurate.
Be aware that even within a single model year and trim, there might be variations. The safest method is physically measuring the speaker opening in your truck's door.