There are probably several ways to go about setting up the page size and cell size but the screen capture should show how things should look.
12.25 X 18.25 is not a normal aspect ratio of a capture from a digital camera so your first step would be to a apply a crop at that aspect ration in the develop module prior to proceeding to the print module.
Also this tutorial should help. Lots of free stuff on Adobe TV.
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To follow on with the answer-
You can type the exact cell size you want- just double-click on a cell dimension and type- 12.25 and 18.25
If your image is not in those exact proportions (aspect ratio) then "Zoom to Fit" and "Zoom to Fill" will have different results.
In my screen clip below I have typed the exact dimensions wanted, but note that I had "Zoom to Fit" active so my image shows its print dimensions slightly different.