I'm not completely sold on the argument that software should aways fill the user's screen. Many websites, for example, don't use the full width of the screen for text, and I think I personally like it that way because I don't have to sweep the entire screen to read. Screens are landscape, rather than portrait like books, so it can be annoying to read overly wide lines of text.
I got so fed up with the fixed size of the Outlook Rules dialog that I wrote a batch script to to resize it using cmdow[1]. (I have over 50 rules.)<p>[1] <a href="https://ritchielawrence.github.io/cmdow/" rel="nofollow">https://ritchielawrence.github.io/cmdow/</a>