Follow Up:

Ok: I probably should have looked it up more diligently in the documentation.
You can increase the fontsize on the fly but with a little hitch; Cmd + but that is actually with the Shift key.
Have a look at Preview: it lets you increase the font size with Cmd = and Cmd +. I find it a bit weird that you have to use the shift key for increasing the size but not for a decrease in the size.
It would still be nice if you can also change the actual font used.