I use gedit on a Ubuntu installation at C++ programming and everything works fine
except for the small default font size that is used. How do I change this, I can't find
any settings (or zoom button) in the gedit menu's? I assume that it should be possible to change somewhere in the Linux environment?

Sorry, it is an option to change font and size under (Edit > Preferences >......) in gedit. It is possible to customize font and size after the default box, which gives DejaVu Sans Mono 8, is unchecked.