6.5 inches
take the inside door panels off and then you can unbolt the speaker from the door.
on the front of the console on the passenger side on the side of the console a small door opens.
Inside of car, passenger side on the side of the console trim casing. Pull panel off to access door lock fuse.
a 6 1/2 speaker fits in the door of a 2006 nissan altima
Wire coat hanger
all of them are 6.5"
rear door-remove the door panel. back speaker carefully pry off grille cover and replace speaker
6X9 in back and 4X6 in front
6.5 inch front and back.
they are 6.5's
Take inner door panel off and the plastic deflector paper and there are 2 10 millimeter bolts that hold the handle on, tighten the bolts and your all set.
http://wiki.answers.com/Q/How_do_you_remove_the_door_panels_on_a_2005_Cobalt"Try to use Chevy speaker adapter plugs so you don't need to cut wires and mess the electric system up. You can get them from Ebay or Amazon