If you followed the instructions on spatial calibration you should have a nice uniform field in Scanbox.  One simple way to measure the resulting spatial resolution and spatial uniformity at each zoom setting is to mount a 40 line pairs per mm RONCHI calibration slide on top of a green aut0-fluorescent slide.  By aligning the slide with the horizontal and vertical axis of the scan you can precisely measure the size of your field:

You can also superimpose them to visually judge if the spatial correction is isotropic throughout the field… which, in our setup it is rather uniform:

The dark side bands are blanked by the pockels cell and their size is controlled by deadband configuration parameter.