For an updated version regarding Sublime Text 3 and a high DPI screen, check out my later blog post which solves it more cleanly.

The sidebar in Sublime Text 3 seems to be happily ignoring my system settings when it comes to font size, at least in Linux. This is a bit annoying when you have a high resolution screen (or “retina” as Apple calls them). There are some solutions mentioned out there on the internet, but they seem to be slightly out of date. Luckily the info contained in them pointed me in the right direction.

Getting on with it! The font size of the sidebar is a part of the theme that is being used. This is in a simple text file which in its turn is in a zip file. Simply find the location of the theme file and get editing. You can find theme files in Sublime’s packages folder. There are two of these, the user ones and the default one. For the user ones (that is, those you installed yourself), you simply click Preferences → Browse Packages… and open the User folder. You will find the appropriate theme file there. Since you installed it yourself, I am going to assume you know the name of the theme too.

For the default one, you will have to locate Sublime’s installation folder which in my case is in /opt/sublime_text/. From that folder, open the Packages/ folder. The zip file in question is called Theme - Default.sublime-package. Now there is just one problem here and that is that this entire folder is likely owned by the root user on your system. No problem if you set up this machine yourself, but if that is not the case, then I believe you can copy the theme file over to the personal folder I mentioned earlier. I have not tested that. If you do have the root password for your system, then you can simply edit the file in question. Note that an update to Sublime Text can undo your changes if you edit the files in the installation folder so moving the theme over may be desired regardless.

With the theme file located, open it in your archive manager (file-roller for example) then open the Default.sublime-theme file for editing. Alternatively, you should be able to unzip the theme file and zip the files back together again when you are done. With the file open, there are two changes to make.

For the first change, find the following section

    {
        "class": "sidebar_heading",
        "color": [130, 130, 130],
        "font.bold": true,
        "shadow_color": [250, 250, 250],
        "shadow_offset": [0, 1]
    }

and add the line "font.size": 26. Substitute 26 with something you find to be a good value, you may need to experiment with this. This first change increases the font size of the headers in the sidebar like FOLDERS and OPEN FILES. With the line added, it should look something like this:

    {
        "class": "sidebar_heading",
        "color": [130, 130, 130],
        "font.bold": true,
        "font.size": 26,
        "shadow_color": [250, 250, 250],
        "shadow_offset": [0, 1]
    }

The second edit to make is to the section just a bit lower.

    {
        "class": "sidebar_label",
        "color": [0, 0, 0],
        "font.bold": false,
        "font.italic": false
        // , "shadow_color": [250, 250, 250], "shadow_offset": [0, 0]
    }

Once again adding "font.size": 26 gives:

    {
        "class": "sidebar_label",
        "color": [0, 0, 0],
        "font.bold": false,
        "font.italic": false,
        "font.size": 26
        // , "shadow_color": [250, 250, 250], "shadow_offset": [0, 0]
    }

Make sure that you place commas in the right places here! Different statements are separated by commas. This second edit changes the sizes of the actual file names in the sidebar.

Opening Sublime Text 3, you should find you have a more readable sidebar. Huzzah! Now to get to fixing the other parts that look tiny…

A commenter notes this creates disturbed colour noise on their Xubuntu 15.04 setup.