Its a bit tricky and I couldn’t figure it out immediately how to do that. To do that go to File->Settings->Editor-> Colors and Fonts->Font. You will notice the font size is greyed out. There you press the save as button on the default or Darcula Scheme. Then press OK, once you do that a copy of the existing profiles is made then you can edit font size.
Hope it helps..
Thank you very much for the information