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.
Note: Now requires OS X 10.10 or later