Increase the font size by 2 pixels or Increase the font size to 2 pixels?

If your font size is 10px and you need to make it 12px, say "Increase the font size by 2 pixels".

If your font size is smaller than 2px and you need to make it 2px (unlikely, but not impossible), say "Increase the font size to 2 pixels".

Note that in both cases you need pixels (plural), not pixel (singular).