A family of Microsoft word processing software products for creating web, email, and print documents.
On a Mac, the number to use is 72 pixels per inch. If you divide 72 into the dimensions shown in Finder > Get Info that will give you the picture width, in inches.
An even easier method is to just open the picture using Preview and then from Preview's menu bar choose Tools > Adjust Size. You can then see what the actual size of the picture is in Pixels, Inches, or Centimeters.
I recommend that you set the picture width to the width of the page. When you use the Insert > Picture function of Word, the image will be automatically scaled down to fit between the margins. This way you have a little wiggle room on the actual size of the picture just in case you want to later adjust the page margin.