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..

If you are new to android studio you might have noticed that there are no line numbers by default. To enable line number you go to file -> settings->Editor and there you can enable line numbers