What size speakers should I order?

[Dead Parrot]

Unless you know the rear speakers are factory, verify the size. 4x10 is what the factory brackets take but if a PO installed aftermarket holders/boxes, decent chance they are 6x9.

Unless you are going for a portable earthquake machine, modern 4x10s are good enough for most uses. Modern speaker tech lets more sound come from smaller packages then was possible 40 years ago. Plus if you really want gut rattling bass, you can always add a sub-woofer.