What does the Raster size option do?

It allows you to set the minimum and maximum dot size and this way control the "tone" of the output. The raster size is set relative to grid size (how far apart the dots are from each other, measured in millimeters).

A raster size range of 0-100% means that completely white areas in source image yield no dots to the output. Completely black areas produce dots so large they fill the entire area with the output color.

If you want to have blank space between all the dots, select maximum value of around 70%. If you want the original image to barely appear, select something like 40-60%.

Source image	Raster size 0-100%
Raster size 0-70%	Raster size 30-70%

Note that in halftone rasterbation raster size is calculated diagonally, since circles are being placed inside squares and they must overlap to fill areas completely. So with max 100% raster size the max raster diameter is √(Grid size² + Grid size²) ≈ 1.41 * Grid size .