ResoMetri lets you drag a cropping rectangle to see if a part of the picture is big enough for printing. Select the units (inch, centimeter, pixel, pixels per inch, points) and the resolution that you want to use. You can also inspect the metadata of your photo. There is a magnifying window in which it's possible to view the details in 100% or 200% size.
RequirementsOS X 10.7 or later
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