How to increase the size of the OS X user interface without lowering the resolution?
There is a feature that's currently experimental called HiDPI that may do what you want. Like the retina displays on the iPhone and iPad, HiDPI mode creates a "logical" resolution that's half the current resolution on your display but uses all the physical pixels to keep the image sharp. This results in all the user interface elements appearing much larger, but with no lack in clarity. For example, if you're outputting to a 1080p TV with a resolution of 1920x1080, the HiDPI resolution would be 960x540, and all the elements on the screen would be the same size as if you were outputting that physical resolution.

To enable HiDPI mode in Mac OS X Lion:


Download the "Graphics Tools for Xcode" download from Apple. This download requires you to enter an Apple ID, which you most likely have and can get for free.
Mount the DMG
Open the "Quartz Debug" app
In the menu bar, select "Window", then "UI Resolution"


Check "Enable HiDPI display modes"
You'll be prompted to log out and log back in. If multiple accounts are logged in, they must all log out for the change to work.
Once you log back in, go to System Preferences, then Displays. You will see the new HiDPI modes there.


For your 1080p TV, select "960x540 (HiDPI)"


P.S. If you're curious what HiDPI mode looks like, the screenshots I've taken are both in HiDPI. Click on them to see them full sized.