Is there a standard size Gobo?

Is there a standard size gobo is a good question. The answer is no. The problem is that gobos differ in size for each lighting instrument. There are certainly some gobo sizes that are more popular than others, but there is no standard size Gobo. Each lighting fixture takes its own unique gobo size, although there are some crossovers in gobo sizes, most instruments take their own unique size.
When sizing a gobo, there are two measurements that are crucial, IA (image area) or VA (viewable area), and OD outer diameter. There are many websites online that provide lists of which lighting instruments take which gobo size and what the exact measurements are.
There must also be a match between the gobo size, and the correct sized matching gobo holder.Like the lighting fixtures, every gobo has it’s own size gobo holder. A good example of this is the ETC Source Four, which can take either a A size gobo holder, or a B size gobo holder, and also accepts a glass pattern holder, so not all gobo holders are the same, or a standard size for each light.
Below are some common gobo size names:
A, B, D, E, Q, M,D....