There are noi ANSI standards for sizes beyond 8X10, with three exceptions: 10X12, 11X14 and 14X17. For all other formats film makers must assume that the stated size of the format is the actual size of the opening in the film holder. That is, an 8X20 format should have an actual opening of a full eight inches X twenty inches. I have seen a few older holders that have openings slightly larger than nominal, but that is not the convention today.
Film is cut just shy of the size of the format. For example, 12X20 film is actually 11 15/16 X 19 15/16, 7X7 is 6 15/16 X 7 15/16. If it is cut by mistake to the actual size of the format chances are you won't be able to load it.
Most sheet film has a thickness of 0.007". So a good appoximation of sheet film for any format could be made by cutting sheets of 0.007" polyesteer to 1/16 less than the nominal size of the format.