it is not possible to do this in outlook 2016 (in the program itself).
most sites will tell you to do this in display settings and change the display ratio to 125%.
you could do that but it messes up a whole lot of formatting in windows and in other programs.
what works slightly better is to go in to settings - ease of access - make text bigger.
move the slider to the right in small increments until you get the size you like.
this may also mess up some other programs as well.