When I have a roll of cut film of any width -- it might be 5 feet or 25 feet -- I simply pull out how much I need. How long that is depends on it it's 16mm, 17.5mm, 9.2mm, etc. It also depends on the camera and how many exposures I want to have on that roll. For example, if I'm loading a Minox cassette, with a pair of scissors I cut depending on the # of exposures I want the strip this long:
I have a few markers. The one I use the most are simply pieces of tape on the side of a cabinet -- that I feel with my fingers -- and SNIP. Nails, tacks, whatever would serve the same purpose.
There are lots of ways to do it. Since my purpose is mainly submini rolls, the lengths are almost always under two feet -- very simple, even in the dark.