How can I change the system font size in OS X?

There's no built in systemwide means of adjusting the font scaling, short of running at a lower than native Resolution. In general, OS X is a fairly poor choice for everyday use at a distance of 10 feet from the screen. That said, you have a few options:

You can increase font sizes on an app by app basis. The main one you'll be concerned with, it seems, is Safari, where you can adjust the default display fonts for web pages under Preferences->Appearance, and on any given page, you can adjust the displayed fonts on any given page you are viewing with ⌘+.

You can also make use of the built in 'Zoom' feature in OS X. To enable this, under System Preferences, go to Universal Access and turn on 'Zoom'. This will enable you to zoom in on a portion of your screen at greatly increased magnification with a keyboard shortcut. Some people love this, but I find it slightly disorienting.

You might also want to investigate whether one of the systemwide 'theming' apps will work for you. The only one I know to work currently with 10.6 is ThemePark. You can probably find more info to assist you in taking this approach over at the MacThemes Forums, but be advised that this sort of 'UI skinning' tends to be frowned on by Apple and break frequently. It's something of a high maintenance option.

Finally, if you're really just concerned with home-theatre type usage here, you might want to look into any of the many FrontRow replacement apps out there that offer better support for Hulu, Vimeo, Pandora et al. Some of the major names in this field are Plex, Boxee and XBMC.

Apple doesn't provide the capability, but third party apps might. You could try TinkerTool which has a Fonts tab that lets you modify system fonts. I haven't personally used it for fonts (although I do use it to change other things like Finder prefs) so I can't say whether it works or not.

There is a way by using command line parameters and it works throughout the system.

(for other readers: the solution works for MacOS X 10.6 "Snow Leopard", but not in 10.7 "Lion" or 10.8 "Mountain Lion")

Taken from here

If you want to increase the size of the menu bar font, you may want to try this. It should work on Leopard.

You will need to launch Terminal (its located in Applications -> Utilities).

Enter the following command.

defaults write NSGlobalDomain AppleDisplayScaleFactor 1.5

Then try launching some applications. You should see the menu bar size change as well as the application.

The last number "1.5" in the command means 150%. If you want to reset back to normal, use:

defaults write NSGlobalDomain AppleDisplayScaleFactor 1.0

Depending on your screen size and preference, you may want to play around with the number.

An unfortunate sideeffect is that everything in the webbrowser gets "shifted to the right" due to the increased size. however the size is otherwise great on our 42" tv, can actually read the top line (apple, file, edit etc).

i.e. to be able to see the "post your answer" button on the bottom of the screeen here I had to use cmd+ "-" to shrink the website.

I could have sworn there used to be a preference to do this before, but I can't find it - perhaps a more able SuperUser-ee will know where it went. There are a couple of things you can do, though:

• Try turning off LCD font smoothing, either entirely or for a higher threshold in the Appearance section of System Preferences.
• Switch to a lower resolution in the Displays section of System Preferences. This will make everything bigger, at the expense of lower graphics fidelity.
• Turn on screen zoom in the Seeing tab of the Universal Access section of System Preferences. Again, this will make everything bigger, but can be turned on and off as necessary.
• Some applications have specific accessibility options: for instance, the advanced tab of Safari's preferences allows you to set a minimum font size that will be used, regardless of what a website requests.