Gobo is used to project image on a flat surface.
For theatrical lighting, gobos are distinguished into several sizes from large to small: A, B, M, D, M, E.
OD (Outer Diameter) and IA (Image Area) are two determinants when choosing a gobo size. OD is the diameter of gobo, the value should be smaller than width of gobo holder, and larger than image area diameter.
Choosing material of gobo depends on whether color is neccessary. Metal gobo is for projecting white image on black background. Glass gobo is for projecting colorful image, but it's easy to be ruined.
Normally, it doesn't recommend gobo image area size larger than 32mm, it might cause fuzziness around edge of the image, reason is shortcoming of ponit light source optics: