What size speakers fit a 1999 Mountaineer?

I have a 1999 Mercury Mountaineer with the Mach sound system and different websites tell me different sizes of what speakers fit. All of them tell me that it take 6x8 but some say that the Mach sound system allows for 5-1/4 component speakers. I don't want to buy anything until I'm sure what fits. If anyone can help me with my problem that would be great. Thanks.