How to increase code font size in IntelliJ?

It's as simple as Ctrl + mouse wheel. If this doesn't work for you, enable File → Settings → Editor → General → (checked) Change font size (Zoom) with Ctrl+Mouse Wheel.

It is possible to change font size etc when creating custom Scheme using Save As... button:

While I was waiting for someone to respond, I looked around a bit more and found the answer.

Navigate to Fonts and change font to whatever size you'd like

To change font size using the keyboard

In Windows or Linux press Ctrl+Shift+A

In MAC press CMD+Shift+A

In the popup frame, type Increase font size or Decrease font size, and then click Enter.

Font grows larger or smaller.