Suppose I'm developing an application with a simple graphical user interface, like a basic text editor. How do I choose a good default size of the application window? Should the height of the window be a percentage of the height of the screen? What about the ratio between the width and the height? Or should I just go with something that looks good?
Don't go with something that "looks good", go with something that meets the needs of a typical user. For an editor that probably means a width of 80 or 100 characters for the text widget, and a height of perhaps 50-100 lines.
At the end of the day, your goal is to meet the needs of the end user, and different users have different needs. For example, hackety hack serves the absolute novice so their default screen is small, perhaps so it doesn't seem so intimidating. Something geared toward professional developers, or for languages that are particularly verbose, might require a much bigger window. An editor for editing tweets would likely be three lines tall.
Bottom line: meet the needs of the end user.If you don't know what their needs are, figure that out before going any further.