my girlfriend want to put these 20 inch rims on her 06 malibu that are off a 04 impala, would those be too big or would there be any problems with that ?

 

Not sure if this will help, but if you go to www.tires.com there is a place where you can select tire/wheels up to two sizes over stock. They show the tires size, which of course requires rims to match.
The other thing to consider is the offset on the rims. You might be able to get this data doing a search for rims from the 2 vehicles involved on ebay.

 

well the largest i have seen is 18inch rims so im not sure, i know the impala has a longer wheel base, just still not sure