* Check your owner's manual: This is the most reliable source.
* Look at the fuel filler door: Sometimes the size is printed there.
* Check the fuel tank sticker: This is usually located under the hood or sometimes on the driver's side doorjamb.
Without knowing the specific trim level (SL, SLT, etc.) of your 1999 Durango, it's impossible to state definitively whether it's a 20 or 22-gallon tank.