As best I can tell there is no way to change the default subscript/superscript size. As you did, I found directions to search for a particular kind of formatting and replace all text with that format, and directions to create new styles and save them. So I created new styles with the subscript and superscript sizes and offsets that I wanted, and made new keybindings for them. I also created a keybinding for the default style so that I could turn off subscript or superscript with keyboard shortcuts. I saved the keyboard configuration. It worked great for the current document, but then none of the new styles or bindings automatically applied to new documents.
I had to create a new template and make that the default template, otherwise it reverted to the standard defaults without access to the new styles and bindings. But I still haven't figured out how to create a new template that has no text in it AND preserves the default font size (12 pt) instead of the font size I happened to originally save my template as. And the template has whatever text I didn't erase. I'd offer a more step by step answer, but it took me a long time to piece this together on different days, and I can't easily reproduce all of the web pages I looked at for instructions. I hope the outline and keywords can be of help.
I want this, too, but I can't upvote it since I don't have enough points.
Me too. Style sheets allow you to change paragraph parameters such as font, spacing etc., as well as character parameters, but not default superscript formatting. I've created a custom style for the superscript, e.g. red, Arial 18, but in order to apply it, I'd have to select each and every superscript then apply the style (find, and find and replace do not work for superscript, e.g. not found). I'd just like it to be that size and colour every time I use Ctrl+Shift+P.