Ubuntu – How to resize the System Settings window

This is something which is managed by your theme. The reason for this is because themes use bitmap images which are not scaleable so for each button size a theme author must provide the images which are pre-scaled to that size. I'm not 100% sure but I believe this is something newer themes may be able to get around by using SVG images (but either way this setting will still be in the same place)

If you don't see an option to alter the button size in your theme 'Window Decorations' tab then it won't be available. The best way to get the button size you want is probably going to be to choose a theme you like.

Another alternative which you may or may not prefer is to adjust the DPI settings for your desktop. Unfortunately, you haven't mentioned the desktop you use so I can't really help anymore unless I wanted to list instruction for each desktop (which I don't). If you manage to find this setting though it'll allow you to effectively 'zoom' the entire screen. This will enlarge everything (not just the buttons). Personally, I like to do this because it makes everything nice and big yet the resolution stays high so it is still nice looking.