ResoMetri is a tool that enables you to quickly check if your photo is large enough for printing. You can drag a cropping rectangle to see if a part of the picture is big enough for the purpose. Select the units (inch, centimeter, pixel, pixels per inch, points) and the resolution that you want to use. You can 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.