If you are using one of the web-browser based releases (hosted or local) then you can use the zoom feature that is build into most modern web-browsers. If you are using the install-able executable release then you can use the zoom or Accessibility features built into most modern operating systems.
As far as I know there is no font scaling feature built into the Twine 2 application itself, although you can temporary(*) change the font-size of main section of the Passage Editor in any release by using the Web Developer Tools (of either the web-browser or the executable's web-kit) to Inspect the HTML that makes up the page and then adding custom CSS like the following to it.
.CodeMirror {
font-size: 1.5em;
}
(*) Any CSS you apply this way should keep working until you shut down the Twine 2 application.