I got a set of 15x15cm (6x6in) enlarger filters which I (obviously) didn't think to check if they would fit my enlarger (Lucky, Japanese brand)
The holder itself is about 11x11cm but the area inside for the actual filter isn't square and closer to 11x10cm. I saw Ilford has a 89x89mm filter set but that wouldn't quite fit the holder either although definitely covering up the negative area.
Is there such a thing as standard filter holder sizes or do people simply buy large filters and cut them down ? (which seems like a waste)