Trying to create elements in the GUI editor I see nowhere I can simply specify the size of the font I want to be using. To do something like change it's color I have to enter "code" but I haven't found anywhere that lists these "codes" I'm supposed to be using.

Something like this it was I believe...

Search for text.code in the Header Browser tool. Acceptable codes and example usage are written in the comments.

Actually, just hover over the Code input box in the GUI Editor. A tooltip appears with the header comments displayed.

Size is determined by TextDS. Either create a custom font using the Font Maker (down-arrow at upper-left corner > Font Maker) and use that or scale the default TextDS via code (e.g. Text_ds.scale = Vec2(0.1f, 0.1f); or Gui.tds_text.scale = Vec2(0.1f, 0.1f);).