Stock wire gauge is about 18.

On my radio, I have adjust the sound to the rear speakers more, because the front Sony'* (90 RMS, 120 Max) become overpowering without much volume. The Rears also get very loud (Lightning Audio 100 soming RMS, 225 Max) with the HU turned up to about 3/4 volume.

The difference between 16 and 18 gauge speaker wires in a car stereo stystem is insignificant. You won't notice any difference whatsoever. For any practical car audio application, the stock speaker wires will always be just fine. One could come up with a theoretical situation, like "If I hooked up an amplifier that could drive a signal with 2000 volt maximum amplitude into a 2-ohm speaker load, I'd be sinking 1000 amperes through the wires, and the wires would incinerate themselves" -but that'* never really going to happen, so this is just not anything to worry about.

Of course, replacing the stock wires with larger gauge wires will also not hurt anything. So if you have time and wire and it makes you happy, then go for it.