No need to change to big speakers for the rear coz rear speakers are not important. They're there just to provide additional ambiance but should not be the main source of music in your car. Focus on the front speaker instead, it has to be superior and more powerful than the rear coz the front is where the sound staging will come from, just like when you listen to a concert. If you have the rear more powerful then the sound will be pulled to the rear hence will ruin your staging.
If you want more bass then better just add subwoofer in the trunk. If you have limited budget then can just add under seat powered sub (choose 10inch one) then will add bass more substantially without ruining the sound staging. You'll also feel the bass more (thru vibration under the seat). But you'd need to fork out at least rm400-500 for that.
If your rear speaker already spoiled then just leave it. FYI some audio purist even prefer not having any rear speakers! If you still want to replace them, then just buy the same size and cheap ones.
