Some programs have a zoom to scale function that requires training the program to know how large their pixels are, so they sometimes do things like ask what size a specific line is, so you take out a ruler and measure it.
A better way of doing that is by using the ISO 7810 standard to your advantage. Credit cards and most ID cards are all the same size. If you can indicate what size that is, you can scale images to the correct size without much effort and it works internationally etc.
Took half an hour to figure out a good standard candle but it turns out that international banking demanding a standard is actually quite useful.