I cut down film all the time, most of it 9 1/2" aerial film on long rolls, the stuff is quite thin and tricky to deal with but once you get the hang of it, it's easy.

I use a craft knife and always cut at least two lengths at a time, usually four to six. I have a board set up for cutting, that has a metal strip taped down as a back stop, this gives me my film width, I'm usually cutting 6 foot long lengths. The back stop is set to 1mm less than the width I want, I lay the film down and push it against the stop, I always have the top film facing down and the others below facing up, never have problems with scratches or damage.

The I lay down another metal strip over the film and tape it to the edge with 1mm sticking out, this is my cutting edge and I run the knife along the metal strip as a guide. I hold the knife underneath the board, blade facing up and carefully run it down the length of film making sure the blade touches the metal strip all the way.

It can be tricky at first and you have to be aware of where your free hand is all the time, I've never had any accidents or even close shaves.

You can also do something similar by using a board with a back stop and two front guides fixed in the right position at either side of the film, push a steel ruler against the guides and run your knife along the ruler. The guides are fixed at the correct length minus ruler width and you can run a groove through the board for a gap for your blade to run along. This is probably the easiest way for cutting down small film, just remember to do two at a time, emulsion facing in so you don't damage it.