The 2015 kia optima lx comes from the factory with a sound system that included 6-¾ full range speakers in the front doors, a single 5-¼ speaker in the center of the front dashboard and another set of 6-¾ full range speakers located in the rear doors. some models will also come with a single 8 full range subwoofer that is located in the center of the rear deck. you will most likely need to buy 6-½ replacements since the 6-¾ size is less common. to offset the size difference you will need to use a speaker adapter which mounts to the factory openings in the front and rear doors and makes the hole ½ smaller so the new speakers fit properly.
The 2015 kia optima lx features 6-3/4" speakers in the front and rear doors, an 8" speaker in the center of the rear decik and 3-1/2" speakers in the dash corners.