What is the average size of a residential solar panel?

In Australia, most “residential” panels are between 300 and 400 watts. 400 is about the largest size that can be easily manhandled on to your roof.

My roof has 6 x 300 watt panels on the west side (older system) and 19 x 350 watt panels on the east side (new system, with battery storage).

In Australia, solar panels on roofs are so popular that electricity providers are now recommending east and west mounting rather than north facing (for the Southern Hemisphere) in order to spread the feed in power over more hours per day rather than just the 5 hours or so around midday.