Mark,
First, the print driver willl not let you create a paper size which is wider than longer.
"The width of a paper size should be equal or smaller than its length. Click OK to let the driver interchange the values for you (remember to select the landscape option to get the desired orientation) or click Cancel to modify the values yourself."
So, although you might want a 24"Wx18"L, the print driver will change that to 18"W x 24"L. But, now you can print this in Landscape mode and the printer will automatically print 24"W x 18"L reducing the amount of paper required.
Now, what you get printed depends on the settings in the "Layout and Margins option" which is accessible from the "Margins/Layout" button on the "Paper/Quality" tab. You probably have the "Print with margins" option checked but now look at what Layout Options you have set. These will affect the actual printed size of the image.
Pierre