Sublime Text 2 how to change the font size of the file sidebar?

Select Preferences / Browse Packages…, and go to Theme - Default directory.

Open Default.sublime-theme with your editor and search for sidebar_label string. You should find something like:

{
    "class": "sidebar_label",
    "color": [0, 0, 0],
    "font.bold": false
}

You can add here the font size you prefer:

{
    "class": "sidebar_label",
    "color": [0, 0, 0],
    "font.bold": false,
    "font.size": 14.0
}