What do the different marker sizes mean?

Marker Size pop-up menu in Variable Settings:

• 0 – 1: Marker is displayed as a single dot.
• 2 – 3: Marker is displayed as a small rectangle.
• 4 – 6: Marker is displayed as a 2 pixel object.
• 7 on up: Size of marker increases 1 screen pixel in each direction for each group of three.