I have cut prints from many boxes of 50 inch and 30 inch rolls.. other than a dispenser that cuts perfect cuts, here is how we do it. I SUSPECT YOU ARE TALKING MURALS HERE AND NOT SMALL PRINTS.

lay the box on the floor. determine how much paper you need to cut with a tape measure, put the end of the tape on your paper box and measure on your body the length you
need. Remember the point where ie your belly button, your neck , your nipples..

then pull the paper emulsion towards you up to the point on your body, gently roll the paper back over the lip and cut along the edge of the box with a good olfa knife.
close the box and put the paper on the wall or in your easel.

this is a very tried and true method of cutting rolls and in many shops the only way the technicians will work. a rotary trimmer with flat surface is not an option for murals that I would recommend with this cutting from the box so simple and efficient.

remember I am talking murals here and not a small prints.