Hello, I have a 2004 Malibu. I bought it with 15 inch rims, and I was wondering if 16x6.5 inch rims would fit on it properly, or if they would be too big.

 

Possibly. It depends on the offset. You need to be mindful of interference with the brake system, suspension and steering linkage. The most fool proof way is to just dry fit them and test them.
You should also match the overall tire diameter to the factory size.

The LT trim Malibu had 16" wheels, so the easiest would be to find a set of those.