Why there is no such thing as a standard size gobo?
People often say they would like to order or make a standard size gobo. The problem with this is statement is, that there is no standard size for gobos. There are certainly some gobo sizes that are more popular than others, but there is no standard size in the Gobo world. Every lighting fixture that projects gobos, takes its own unique gobo size, although there are some crossovers in gobo sizes, most instruments take their own 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 gobo holder. This is also not a standard size. 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 a glass pattern holder, so not all gobo holders are the same, or a standard size.