Page title layout is defined by the "elgg-heading-main" class which is defined in views/default/elements/typography.css (it might be that your theme has this css file also so the theme's version would override the core file definitions). Changing font size can't be done using the font-size attibute (default size for example in Aalborg theme for title font size is "font-size: 1.5em;"). You could add typography.css in your theme plugin and adjust the font size then.

Size 200x200 is the default size for large thumbnail icons in Elgg. I'm not sure right now if Elgg 2.x already allows changing the defined size specifically for group icons alone separately (Elgg 3 will allow it if I'm not mistaken). You could try out the Customsize Groupimage plugin (https://elgg.org/plugins/1905508). This plugin displays the group image on group profile pages in the original size (of the uploaded image). So, you might have to adjust the image size before uploading the image but then no other changes would be necessary to display the image in this size.

Hm, I had the page open already before the reply got posted. With the Subgroups plugin the Customsize groupimage plugin might indeed not work. I can't say at the moment what would have to be changed to get it working (if possible anyway easily). I also won't have time to check this for the foreseeable future.

No problem, just answer me saying whether or not you know a solution already helps me. Thank you!