Speaker size??

[70 Charger RT]

I have a 5.5 watt AM / 8 track with an original speaker fader.  The original front speaker was measured at 8 ohm.  I'm not sure what the back one was as it was missing.  I bought two small Clarions SRG1022R speakers (4 ohm each) and hooked them in series to get the 8 ohms to replace the front speaker.  They mount nicely in the original front center bracket.  These Clarion speakers are 30W.  Question is: Do I select the same speakers for the back as I did for the front?  I know it should be 8 ohm for whatever I pick but I'm not sure what watt speaker would work best.  Should they watts match front and rear speaker?  Any audio experts out there?