Rear speaker sizes

[Delorean17@xxxx]

Hello,
I am in the process of replacing all the speakers in my car. The other 
ones are rotted out. I would prefer not to use 4x10 speakers in the rear 
again. what is the biggest speaker I can fit in there. Will a 5x7 fit with 
the right sized magnet? or will I have to do something more like a 4x6? I 
just dont want to change the appearance of the interior so it needs to fit 
back there!
Also, if I decide to put a sub or 2 would it be a bad idea to put them in 
the trunk?

Thanks
David