Does this watch update it’s time automatically in Australia?

Australia must have a service like our NIST to update time. I do not know if they do so I cannot answer your

Located in san diego. using the get feature to get automic time gives me an err message. the signal was l3 which means strong. any suggestions?

Also in San Diego with L1 signal strength and I get an updated signal most nights between midnight and 3am (auto update). Make sure you have set the home city correctly (LAX), so that the watch is looking for the right frequency signal, and try it in a different location in your house. I recall it took some time for my watch to get the first signal - a bit of trial and error on location/orientation. After that, I've been surprised a few times when the watch was in than a less than ideal location, that it still got the signal. The manual has some helpful hints on how to orient the watch Search for "Casio 3193 manual" if you cant find yours.

Is this considered an XL face ?

I am not exactly sure what constitutes an "XL" face but this one is rather large. I am not a large guy but this watch face practically goes from one side of my wrist to the other. Also, if you wear a lot of long sleeve dress shirts you may have an issue buttoning the sleeves. The watch sits high on the wrist. However, most G-Shocks do. Otherwise, this is a great watch. I like the fact that it has a light. I have worn Casio G-Shocks for 20 plus years. They may not be the most stylish but they definitely are the toughest. One of them I wore for 10 years, went through three bands, and finally the plastic bezel around the watch dry rotted and fell apart... It still kept good time. The best hundred bucks I ever spent.

Would this fit in the regular G-7900-2 case?

No case needed. The watch has its own enclosure and works out of the box, including rechargeable battery and automatic automatic time synch from Colorado Mountain broadcaster from midnight to 5 AM, so the precision is by the second.