The very generously sized AvastClear uninstaller GUI is too big for some XP display screens in Safe Mode. The action button can be out of sight and inaccessible…
I have noticed this also quite some time back on my Dell XP. I managed to move the Avastclear screen with the mouse at the very top just enough to see a part of the Button.
Eventually I’ll stop using xp all together 
Unless Avast users have extremely poor eyesight, the GUI text could be reduced in size and still be very readable.
I doubt that reducing the size of the text would result in a reduction of the avastclear window.
In much the same way as you can change the text size in windows and or your browser, etc. it doesn’t change the size of the window.
A greater help would be to allow the window to be resized.