* Front Doors: 6x8 inch (most common) or sometimes 6.5 inch.
* Rear Deck: 6x8 inch (most common) or 6x9 inch.
To find out definitively what size speakers are in *your* 2004 Impala, the best approach is to:
1. Visually inspect the speakers: Carefully remove the speaker grilles (usually just pops off) to see the speaker itself and measure it.
2. Check your owner's manual: This should specify the audio system installed in your vehicle.
3. Look up your VIN: Using your Vehicle Identification Number (VIN), you might find detailed specifications on a Chevrolet parts website or a vehicle history report service.
Without knowing your specific trim level and audio package, it's impossible to give a definitive answer.