With some modification to the speaker housing I was able to fit 5.5's back there. you might be able to go larger if the mounting depth of the speaker isn't too deep.

Note: Be careful with your panel clips. Save them. Take note of what clips go where... (I think there's a few (2) variations behind there). After taking mine off I've never been able to get them back quite the same way (causing some panel squeaks/rattles, etc.)

Best of luck!

-biju.