Your system size can be determined by historical usage, like over a year's worth of power usage. You could also add new, planned EV, like I did to make the system larger. Your installer should be helping you with these numbers. If your system is "getting installed" what was the system size based on? In my case, I had no historical usage for the new home, so I used the SCE recommendation of 2 kWh sq. ft., so my system size is 6.6 kW.
With my Model S, I'm using all the power I generate. Given actual panel output, I can safely add another four panels and be right at the edge at noon facing due south, 180º azimuth. However, you really don't want to oversize your inverter too much. Your installer should know and make the proper recommendation.