Perhaps the format you are entering them in is incorrect.

When using the boxes enter:
font.min-size.fixed.x-western in the first box and 20 in the second box

Do not put in the word "int" and not the "user_pref" part.

You need to change all four of these to get things to work on all sites:
font.min-size.fixed.x-western 20
font.min-size.variable.x-western 20
font.size.variable.x-western 20
font.size.fixed.x-western 20