Many thanks to you Angen, Nescio and Stanislas69 who replied so quickly.
I did as indicated and it did the trick!
For my personal taste I used factor 2 for the gui.scale.
Would be great indeed having these features somewhere. I was expecting to find it under settings.options.graphics in main menu window, but I am just a casual gamer and other place might fit better.
In the meantime, having this information under FAQ would certainly help.
Gonna play and test it further now.
PS FYI forgot to mention that I use Ubuntu as operating system.