Just cut down a 50" to 44" with a chop saw as Mark said. Make sure you don't tape off too much as the tape can lift any coats. I didn't have any problems, but something to consider. A good a clean cut with no loose strings.
Funny enough, the day after I purchased the 50" canvas, I was driving down the street and there were some cabinet workers by my house. I stopped and asked them if they would cut it for me, as I noticed they were cutting thick plywood.
I figured this would be an OK blade for such a job. So I asked them and they cut it.
Paper will be next, and I expect it to be as easy, except I'll have to pull my chop saw out and make sure I have a good blade for it. I wonder, If you go too slow you may get burns?