Calculating the excact size of a PCB from a photo?
... the golden connector is 8.35 cm long.




Figure 1. Using an image editor select tool to get a pixels/cm scale factor.

The image shows a selection tool reporting that the 8.35 cm connector is 194 pixels wide. \$ Pixels/cm = \frac {pixels}{cm} = \frac {194}{8.35} = 23.2 \$.
You can use this figure to scale any other pixel measurements with reasonable confidence on the X-axis. It will work on the Y-axis too if the photo has been taken from an orthogonal viewpoint.

If you were to scale this photo by \$ \frac {100}{23.2} \$ you would have 100 pixels/cm and measurements would be very easy.