5X7 all around
all speakers are 5"x7" - its a special size for cars made by ford motor company
4x6- not upgradable without sheetmetal modifications.
my tire sizes for my cougar are P215/50R16 or go here so you can find your size http://www.dragtimes.com/Mercury-Cougar-Tires-m31-n237.html
On a 2000 Mercury Cougar : Open the drivers door , and on the end of the door ( or the latch pillar ) you will see an information sticker . One of the things it shows is the original size of tires on the vehicle from the factory
6 * 8 or smaller there are few speakers in this size. 6 * 9 will not fit.
According to the 2000 Mercury Cougar Owner Guide : Both the 2.0 litre four cylinder and the 2.5 litre V6 engines came with the Motorcraft BXT-40R battery ( that's BCI group size 40R )
Call your local Auto Part store.
The 2000 Mercury Cougar V6 engine takes a ( BCI Group Size 40 R ) battery The battery dimensions are : Length ( 10.90 inches / 277 mm ) Width ( 6.42 inches / 174 mm ) Height ( 6.88 inches / 175 mm )
2.0 liter ( 121 cubic inch ) DOHC - 4 cylinder engine
fronts 5" x 7", backs 6" x 9"
I believe that there are two speaker grills on the dash they are 3.5 In. Than in the doors its 6in and in the rear it 6x8 but I put some 6x9 in mine!
215/50r16