A simple package that lets you toggle between two font size

Default (Windows).sublime-keymap

[
    /* You can add shortcut : example for Windows */
    { "keys": ["ctrl+alt+shift+keypad_plus"], "command": "reset_font_size_to_user_defaults" }
    /* ... */
]

Preferences.sublime-settings

{
    "default_font_size": 9,
    "font_size": 9,
    "spectator_font_size": 12,
    /* ... */
}

reset_font_size_command.py

# Fork from: http://larsnordeide.com/2013/improve-sublime-text-2-part-1.html
# Store the following command in a file reset_font_size_command.py in you User package folder.

import sublime
import sublime_plugin

class ResetFontSizeToUserDefaultsCommand(sublime_plugin.ApplicationCommand):
    def run(self):
        s = sublime.load_settings('Preferences.sublime-settings')

        if s.has('default_font_size') & s.has('spectator_font_size'):
            if s.get('font_size') == s.get('default_font_size'):
                s.set('font_size', s.get('spectator_font_size'))
            else:
                s.set('font_size', s.get('default_font_size'))

            sublime.save_settings('Preferences.sublime-settings')