Mixed RAM size?

It should work if they have the exact same specs (other than size) and are not overclocked, i.e., they are 1066 or 1333 MHZ at most (I have some doubt that you could run them reliably at that speed). If I were you I'd do some research and make sure that the manufacturer will support that configuration. I haven't seen any test using six 4GB modules and Kingston is the only company that demonstrated such a kit of 4GB single-sided modules running at 1066 MHz. Good luck finding an additional kit later on that will work reliably with one that was bought several months earlier.