In this post I cover how to check the size of a subject from a photo taken at or above life size (i.e. 1:1 or higher) magnification. This information can be useful when trying to obtain an ID for an insect / flower you've photographed. For subjects taken at less than life size magnification, they should hopefully be large enough for you to get a good estimate of the size by eye. Carrying round a notepad and pen with you may be useful to record the size.
If you are using Canon's MP-E 65mm/2.8 Macro Photo lens, this lens records the magnification data in the EXIF maker notes. For other lenses you need to either record the magnification the photo was taken at, or make sure you take a reference photo at a specific magnification.
When calculating the size of your subject from the image, the first thing you need to do is to get the sensor size of your camera. Google is handy here. For my 450D, the sensor width is 22.2mm.
In ExiftoolGUI browse to your image, select it, then in the Maker tab look for 'MacroMagnification' value.
Or in ExiftoolGUI you can otherwise click the Exiftool direct button and then type -MacroMagnification in the pop-up box, and press enter to see the value.
If you are using a Mac or Linux, or use Windows but don't like GUIs, then you can use exiftool from the commandline, the command should be something like exiftool -MacroMagnification _MG_1206.CR2
Your Image processing software may not save the maker notes into JPEGs (e.g. Adobe Camera Raw doesn't), so you have to check the MacroMagnification on the original CR2 file rather than a JPEG you have processed from the CR2.
Divide the sensor size of your camera by the Magnification, e.g. my 450D's sensor size is 22.2mm, and Magnifcation for my image was 2.4x, so 22.2/2.4 = 9.25. This is the width of your image.
Open the image (uncropped) in Photoshop, then go to Image > Image Size.... In the Image size dialog untick 'Resample Image'. Then enter the correct image width you worked out earlier (in my case 9.25mm):
Now you can use the ruler tool in Photoshop to measure the size of your subject. I can see my bug is 3.6mm long.
Hope this is helpful.