It should remember by default. If you are on Plasma, you can do a temporary fix by creating a window rule for firefox and tell it to remember it's size.

Start your monitor before starting your PC!

You can disable the built-in editor! ...but it would not open any scripts in your external editor, which is kind of annoying. It could also disable the debugger, i didn't use the full potential of an external editor for Godot, you'd have to see for yourself on this one!

Damn, they are doing good work so fast!