What size speakers are in a 2007 Pontiac g6?

Depending on if you have the Monsoon sound system or not, it may differ.

If you don't have Monsoon:

Front Doors: 6 1/2" & 1" Tweeters

Rear Deck: 6 x 9

I'm not sure what is in a Monsoon system, I do not have one, but I hear that it has 8 speakers throughout the car.