|Ok, to make it easier, right-click the following link and save as...
then go to Blender, File > User Preferences
Install Addon button (or Install from file...)
Select the py script you downloaded
Enable the addon by ticking the little box to the right of the addon entry
|well, you set a factor and click the rescale button:
100 means 100 pixels of the image will be scaled to the size of 1 blender unit.
If you got two images, 800x300 and 300x400, they will get different scale factors.
800x300: 800 / 100 / 2 = scale of 4
300x400: 300 / 100 / 2 = scale of 1.5
4 / 1.5 = ~2.67
800 / 300 = ~ 2.67
you will get a correct pixel proportion
|i didn't care about unit system yet, tell what you need
Blender uses units by default, you can set it to meter which will change nothing, only imperial will make everything ~3.28 times larger