what determines what size window Safari starts up with?
It is using a much smaller window than in the past. I don't know what I did to change the startup / new window size.
MacBook Pro 16″, macOS 15.5
Safari doesn’t have a fixed startup window size — it just reopens using whatever size and position it had the last time you quit it properly. So if Safari is opening in a tiny window, it likely didn’t get a chance to save the last session. Just resize the window to your preferred size, then quit Safari using ⌘Q. The next time you open it, it should remember that size.