Make Blender usable on high resolution screen

In version 2.8 this setting is now under Edit -> Preferences -> Interface -> Resolution Scale

As of the latest version, the setting name has changed.
File > User Preferences
or CTRL ALT U

And the very first entry at the top:
Display: Scale

Another thing that is very useful is to hold down Ctrl and the Middle Mouse Button to rescale various panels. This is the same for the 3D Viewport when zooming in or out to view a model. The Outliner Area is the only place I've found so far where this does not function yet.

By pressing the HOME key on your keyboard, you can reset the resized panels back to the original default size.

To make the UI elements even larger you can choose Double under the Virtual Pixel mode this option is in User_Preferences->system

Hover the cursor over one of the panels and hit Numpad + or Numpad - keys on the keyboard to change the size of the font and UI elements.