You'll need to rotate that scan to about 359 degrees to level it. Then you can crop it to the exact size you need if you can convert pixels per inch to inches. The size of the selection you'll be using to crop shows in the IrfanView title bar.
You can crop and undo if you want to experiment. You don't have to print to see the size. The actual size of the cropped image (in inches) shows in the Image/Information box.
"If you look at the sun without shielding your eyes, you'll go blind.
If you look at the moon without covering your eyes, you'll become a poet."
--Serge Bouchard