Thank you @Vinod Kumar!
For future reference
I needed to play around with the settings a bit, this is what turned out to work for me:
QT_AUTO_SCREEN_SCALE_FACTOR=1 QT_SCREEN_SCALE_FACTORS=0.5 /opt/Enpass/bin/runenpass.sh %U
It also does not work when I add it to my .bashrc file, it only works when specifying on the command line as above or adding it directly to the runenpass.sh script.