How to scan Item and get correct measurement?

Hi. I'd like to know how I'm going to scan and item on our printer and then get the exact dimensions of the item. For example, If I scan my iPhone, how am I going to get the exact dimension of it and turn it to vector lines. I want it to be exact and precised.

I may be wrong, but couldn't you just use the 'dots per inch' setting on your scanner to get the physical dimensions of the scanned object?
For example, let's say you scanned something at 600x600 dpi. First, you edit the picture to crop the picture so that there aren't any extra pixels on the width and length of the object. After that, you take the number for the length of the picture (in pixels) and divide that by 600 to give you the length of the object in inches. Then you do the same for the width. You might not get the EXACT physical dimensions, but I think that it may be as close as you can get with a scanner.