Using Recommended Resolution reduces Viewable Size

Hi

I recently purchased a Dell UltraSharp U2312HM 23-inch Widescreen Flat Panel Monitor

• Max Resolution: 1920 x 1080 / 60 Hz
• Viewable Size: 23"

When I select setting to the Recommended resolution at 1920x1080 (which is the same as the Max Resolution), the Viewable Size reduces to what would fit within a 21-inch monitor.

And when I select to a lower resolution at 1680x1050, then the Viewable Size is full screen of this 23-inch monitor.

I would expect that the Recommended resolution (1920x1080) would match whatever the full screen Viewable Size of a 23-inch monitor

In my next purchase, how can I determine if setting a monitor to its spec Max Resolution will actually fill it's spec Viewable Size?

Monitor was running off of a Inspiron 17R laptop, new with Windows7.

Thanks

jeff in seattle