Which rear speakers to use?

[Studebaker]

I am installing a radio out of a 02 Saturn Vue. Part # 21025330.

Front speakers will be Kicker KS40's with 30 watts rms.

Which speakers should I use in the rear? I would like to keep them in the stock location. I can fab mounts.

Will I hear a difference by using 5.25 speakers?

What have you guys done?

Thanks David