The best way to get accurate measurements is to go to the site and actually take accurate measurements. I understand this isn't always possible and I've been in the same boat you are in, so we do what we can with what we've got.
I never go by the "average sized brick" or the "average sized door"; this is a quick path to costing yourself a lot of time and money re-doing projects. Don't guess.
That being said, if you know the exact size of something in the picture and can compare that to something that is the same distance away from the camera, you can use that to create a "ruler" that you can use to scale things up. Ideally, you'd want as big of a "ruler" as you could find, but in this case it looks like you're stuck with the bricks.