Truth is I forget the details right now, but from testing around it seems like new window size is determined by two things:
Menu item Format > Line Wrap > Column ... the larger the column number the larger the initial window with will be. The new window sizes itself to fit the chosen number of columns width.
The second factor that determines window size is the font used in the theme. A smaller font will create a smaller window, since it requires less space to create a window that will contain the small font at the desired number of columns.
It's not super obvious... but the way that you change the font in the current theme is to:
- Create a new plain text document
- Choose menu Format > Font > Show Fonts
- Select a new font
- This new font should be applied to the current document, but also for future new documents both plain text and rich text.