It's sized at 1152x864 and is downward compatible with lower desktop resolutions. Just set it to fit to your desktop.
Here's how to use the wallpaper:
Windows computers: Click on the link above for the wallpaper then right click on the photo and select "Set As Wallpaper." In some browsers, it'll say "Set as Background."
If you're using the newer OS-X, then just follow the similar directions above. Else:
FIRST click the link above to load the photo, and then....
1. From the browser menu, select File and then Save As..., select a location on your Macintosh to save the picture (for example, on your Macintosh HD disk, or in a folder therein)
2. Click on the Apple Menu in the upper left hand corner of your desktop, select Control Panel and then Desktop Pictures
3. In the dialog box that pops up, click on the Picture button (as opposed to the Pattern button), Click on Select Picture and select the picture you saved earlier
4. Click Set Desktop