Hi Jordi,
For more specific changes you need to modify the style file
click on the "Format" menu -> "Edit Stylesheet"
A new window pops up. On the top left click ""Choose a Style" combo box
Select for example "Input"
A new cell appears. Click on its cell bracket on the right and then "Format" -> "Option Inspector"
On the search box write FontSize and then choose the size you want
Thats it - it will effect both the printing and how it appears on the screen
You need to repeat this for each cell style you want to modify. However, it is best to find a prepared style that meets your needs.
I wonder, the default font size for "Input" is 13 points. Isn't this enough?
best
yehuda