The 2013 kia optima has an oil drain plug size of 16mm. but before changing your cars oil at home you may want to consider bringing it into a mechanic.todays car design and equipment does not make it too easy to change the oil at home and especially the filter. it can be very tricky and might need special tools, which vary from manufacturer to manufacturer. in certain cases, accessing the oil filter may entail removal of other components from the engine bay or sump guard. so unless you are experienced with working on your car yourself, its a job best left to the professionals. what you can do easily do at home is top-up various oils and fluids if their level has dropped down.
The 2013 kia optima has an oil drain plug size of 16mm