Your screenshot does not clearly show a problem. If it is already “too big” why would you then try to use it full screen? And what is “this” in “this appears?”
You did not mention whether you tried the command line options I mentioned in my first reply above. I assume you did not. Why not try suggestions provided before posting for more help?
Try running shotcut with command line option --QT_SCALE_FACTOR=1
See related
Given the comment in that thread from pete about KDE, perhaps Qt5’s HiDPI behavior is not working well under GNOME.