As far as I know UI only expands if you have it set to stretch.
Using 1024*1024 would be weird since Phones arent square. I use 1080x1800 for my “full screen” components, and work from that. But there is no “magic number”. This is not a Unity or 2d specific question, so just google your way there, plenty of info on this
Best practice is to provide assets for each resolution but I’m always too lazy for that.