Choose the size in the drop-down menu. You also can directly modify the size from a list: by keeping the key [Ctrl] pressed, use the mouse wheel upwards to enlarge the font or downwards to reduce it.
Modify the font used in the grids on the screen
Go to the menu My preferences > grids > Contents of the courses.
In the frame on the bottom, select the type of grid (timetable, or schedule) in the drop-down menu and then select a font and its size.
Modify the default size of the font in the homework notebooks
Go to the menu My preferences > display> Homework notebook.
Select the font and the size that will be selected by default during the entries in the homework notebook.
Note: the font and its size remain modifiable when entering.