* The owner's manual: This is the most reliable source.
* The fuel filler door: Sometimes the size is printed on a sticker there.
* A vehicle identification number (VIN) lookup: Many online services allow you to input the VIN and get specifications, including fuel tank capacity.
* A reputable online parts website: Search for parts for your specific year and model; the fuel tank specifications might be listed.
Keep in mind that even within a model year, there might be slight variations based on trim level or optional equipment.