go to option - customize emacs - top level customization group
scroll down to Faces and press on go to group
scroll down to basic faces and press go to group
scroll to Default face: an press on show
from there you can change the fonts size.
after you changed the fonts press on state and choose save for future
sessions
go up again and choose save for future session the press on finish.
press on finish until you arrive to the regular screen.