It's part of the developer tools, which you can install from the leopard DVD. Once you've done that, you'll find "Quartz Debug" in "/Developer/Applications/Graphics Tools" with the finder. Open that, and select "Show user interface resolution" in the tools menu.
You should then get a slider that adjusts the resolution. 1 is 'normal', 2 would be double and so on. The catch here is that you have to quit and reload any applications before it has any effect. If you want the system to use the setting, press alt-command-escape to bring up the 'force quit' menu, click on finder, then click 'relaunch'. There you go! Some things work, some things don't. I'd click on the 'Restore scale factor...' tick box on the UI resolution window so it goes back to normal when you quit, so you can test it (you'll still need to relaunch finder to get back to normal).
Here's mine running at 2.0x: