what is a good size and brand of speakers to replace the rear ones or all

 

Not sure. I'm planning to replace mine very soon. A good starting point at least
to see some of what fits is to hit the crutchfield site...

 

If you're staying in the stock position you can use a 6.5" speaker of a pretty hefty size. You'll want to replace the front one(s) as they're the two you ought to be hearing the most. The rears are just there to fill in a bit.