I found this link for editing the scale factor of the display: https://www.enpass.io/support/kb/troubleshooting/enpass-looks-too-small-or-big-on-my-display-how-can-i-fix-it/
At first I had a problem of understanding what means the saying ''Please open ~/.profile in your favorite editor and append these lines...'' After some seeking I found the home/user/.profile file in Linux Ubuntu and edited it as advised in the linked instructions, i.e I added these 2 lines to the .profile:
export QT_AUTO_SCREEN_SCALE_FACTOR=0
export "QT_SCREEN_SCALE_FACTORS=1;1"
After checking the outcome on internal and external monitors I made further editing of .profile with the following command: export "QT_SCREEN_SCALE_FACTORS=1.3;1" .
Results: After this command a.) both Enpass window size and font size are OK in both internal and external displays and b.) the square for 'Return to a smaller window' is in place in the right upper corner.
Summa summarum, now all OK with this question.
Thank you for the advice !
Edit of 22.2.2022: The question about location of .profile file was solved for OS Linux Ubuntu 20.04.3 LTS, but remains open for OS Fedora 35 and Manjaro 21.2.x.
