Hello and welcome to Just Answer. Unfortunately manufacturers have made it a little harder to change tire sizes now than it used to be. There are a lot of additional things that have to be considered when changing tire sizes. ABS, traction control, and Instrument panel calibrations are all set based on a specific diameter of tire. So when changing the wheel size or tire width you have to maintain the original tire diameter. With that being said I have found a good resource for this kind of thing to be http://www.tirerack.com/tires/SelectTireSize.jsp?autoMake=Chevrolet&autoModel=Malibu+LT&autoYear=2006&autoModClar. I have loaded your vehicle in the above link, just click the optional size button and it will take you to the correct size tire for the wheel size you choose. It's important to maintain the overal tire diameter so don't deviate from that or you will run into other problems.
You can put an 18 inch wheel on with no problems. Of course an 18 inch wheel has to have an 18 inch tire installed on it. What I meant as far as diameter I was talking about the overall diameter of the tire/wheel assembly. According to the specs for the 215/60/16 original equipment tire the overall diameter is 26.1 inches. So to use an 18 inch wheel/tire combo you need to find a tire that yields the same 26.1 inch overall diameter. So for example you might then have a 225/45/18 tire to yield the same overall diameter. As long as you maintain that original diameter then you will not need to have any calibrations changed, so you can have any tire shop install them for you.
Oh okay, now I see. With the tire/wheel assembly just be sure to have a 26.1" overall diameter. I get it now. Thank you for your answers, they were VERY helpful. Sorry about not replying sooner, this is my first time near a computer in about 4 days. Once I get everything in order and I get the wheels and tires put on I will be sure to let you know how everything goes. Once again, thank you for all of your help; I really apreciate it.