A quick Photoshop Ruler Tutorial for the forum.
1. Take your photo and find a reference with a known dimension.
In this example, we'll say the edge of the mirror housing is 900mm.
Click on ANALYSIS>RULER TOOL.
2. Using the ruler tool, take a measure against your known dimension,
then select ANALYSIS>SET MEASUREMENT SCALE>CUSTOM and fill in the details in the dialog box.
Because you have already made a selection using the ruler tool, click on your selection and enter the details for the known dimension - in this case 900 and mm.
You can now use the ruler tool to measure the edge of an object. The angle and measurement shows in the toolbar under the Analysis button. You can also use the STRAIGHTEN button to rotate the measured edge in the image to vertical or horizontal (whichever is closer).
Where there is distortion due to perspective, you may need to recalibrate (say, using the back edge of the screen for objects in that region of the photo) to get an accurate result.
Best practice on some photos of objects you can later measure and confirm to get the hang of it and you'll master on screen measuring in no time.