Measuring a cartridge filter element when you dont have the exact model is the best way to identify it. There are a few important measurements, and they need to be as accurate as possible.
Total Length: Length of the filter, excluding the handle. You can generally also exclude the spigot or thread.
Diameter: This is the width of the filter.
Spigot Diameter: You may have a male threaded, or non-threaded spigot to locate the filter.
Hole Diameter: If you have a hole, rather than a spigot, measure the diameter of the hole.
Thread / Hole Type: Generally these can be a hole, a fine thread, course thread, or camlock style fitting.