You could probably squeeze 6in or 6.5in 2-ways back there without modifying the rear tray or requiring spacers, but you wouldn't be able to use the factory grilles (you would use the ones that come with the speakers instead.)
I would question why you would want to replace them in the first place - it would shift your sound stage back towards the rear of the car - which only benefits your rear seat passengers, not you. And without going to a 6x9in speaker you won't get much of a bass increase if that is what you are after. (6x9's = idiocy in my book as they are simply a cheap option for the bass brigade who like lots of sound but don't care so much for quality) If you want more volume and better quality, you are better to replace the front speakers with some really decent component speakers (a component speaker is one where the woofer and tweeter are separate units), and then maybe consider amping them off a very small amp. (or a bigger one if you like, but I assume you just want better sound, not a big system). Then if you want bass, hook a sub (even a small 10in would do) to an amp in the boot, remove the factory rear speakers to allow the bass to escape into the cabin more effectively, and viola! an awesome sounding system for very little outlay.
As they say, you don't stand with your back to a concert, so why do it in your car? But its up to you really.