ubuntu has a number of desktop environments....if it is "out of the
box", then it most likely is GNOME. That means this may or may not work,
depending on whether it supports KWIN (I have KDE).
On the top left of the MD windonw, right-click on the MD icon, and then
select "more actions". There you will find a couple of options...all
seem to lead to the same menu where you can change the font size for
this specific window.
Just remember to note what you have changed. Remember to take a backup
copy of your MD data (File->Export backup).
It is hard to help if I don't know the desktop environment that you
have. I am going to assume [out of the box/default installation of]
Ubuntu 18.04.3 and the GNOME desktop (the icons for apps are listed on
the left side of the screen.
I am also going to assume that you are using a desktop, and not laptop
and a display of 1920x1080 (see System-> display system information.
Other specific information can be found at the referenced article.