How do I force Sublime Text to indent two spaces per tab?

If you want it for all files, go to Preferences -> Settings - Default/User. But as several comments below indicate, Syntax Specific settings can limit it to just the languages you choose.

To limit this configuration to Ruby files, first open up a Ruby file in the editor, and then go to Preferences -> Settings - Syntax Specific. This should open a settings window named Ruby.sublime-settings

Save these settings:

{
  "tab_size": 2,
  "translate_tabs_to_spaces": true,
  "detect_indentation": false
}

Repeat for any other syntax types by opening a file of that type and going back to the preferences to open the correct preferences file for that syntax.

I have edited this to include the "detect_indentation" line per the requests in comments; I previously used the Default/User to set my tab size, and have not needed the tab detection, but whether that is due to the global config or due to the fact that I have rarely opened files with tabs, I do not know.

Restarting should not be necessary, although in some instances it can be.

If you want to force your chosen tab setting, ignoring what's likely already going on in the file, then you should include detect_indentation in your configuration (your User settings or your Syntax Specific settings, depending on if you want it global or per-filetype):

{
    "tab_size": 2,
    "translate_tabs_to_spaces": true,
    "detect_indentation": false
}

You can also do this with the text link in the bottom bar of Sublime Text 2 ( On the right side ) that says "Tab Size 4" by default, click that and a window comes up with options to set the tab size from 1 space all the way up to 8 spaces and includes options to convert tabs to spaces and spaces to tabs.

Looks like this:

Can I suggest EditorConfig? There is an extension to autoload and apply the .editorconfig file. Then just create one in the root of your project.

.editorconfig

[*.rb]
indent_style = tab
indent_size = 2

This way, your settings are project-specific and file-specific if you use different styles for each project or language.

---

Here is what my own .editorconfig looks like.

# editorconfig.org
root = true

[*]
indent_style = space
indent_size = 2
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true