Setting Pixel Size

The pixel size of your Analysis Surface and Detector Entities are set in the areas highlighted in the screenshots below.

The pixel size will be the full width of the detector divided by the number of divisions, which in the first case below would be 1/21 = 0.0476mm. If you are using the Analysis Surface, then make sure to uncheck the "Autosize to Data" option to get explicit control over the grid widths, otherwise FRED will autosize the grid based on the footprint of the ray bundle.