Is it possible to change the font size of the project panel?

This worked for me in IntelliJ 14.0.3 Community Edition in Ubuntu:

File | Settings | Appearance & Behavior | Appearance

And then tick on:

[ ] Override default fonts by (not recommended)

    Name: Ubuntu       Size: 14

There you can choose the font and the size of everything: menus, Project panel, Editor, etc... For me, on Linux, it is fine to set it to 15 (default is 14), and then adjust the Editor font alone under:

Editor | General

Mouse
  [ ] Change font size (Zoom) with Ctrl+Mouse Wheel

UPDATE-2016: it is possible, see this

It's not possible, unfortunately, please watch/vote for the bug.

UPDATE: In the later versions global font/size setting under Settings (Preferences) | Appearance also controls the project view tool window font, but there is still no way to control the project view font individually, without changing the font/size of the other UI elements.