What Chan said. At the lab I worked at we had a number of such easels, which were used under normal enlargers for quantity printing. You print the batch and then feed that whole portion of the roll into the processor. Prints then would be cut up after they were developed and dry. It might take some looking, but I bet you could find a used one cheap from one of the many labs that have shut down their darkrooms. Hopefully not everything related has been trashed.
I do the same as Mark and cut 50 or so sheets are a time on my rotary cutter. Since I like borderless prints I usually don't worry about the size too much as long as the sheet is "long enough." I then cut the print to size after I am finished processing it. The only real difference in my process is that I unroll, say, 20 feet from the roll and put the rest back in the box just in case lights would get turned on so only that much paper would be spoiled. I don't worry about any waste that much - I just use it for smaller pieces. After cutting the sheets are stored in a paper safe of a light proof bag.
I'm cutting down 40-inch-wide rolls in total darkness using a big Rototrim I specially modified. It's perfectly safe and does a clean job. Guess you could invest in some night-vision goggles if necessary.
12"x18" My favorite
24"x36" For large prints
cut down 11x14 and you get two test strips. You get 3 from a 16x20 sheet. Make paper stops from mount board and hinge them with tape and flip them up,down, The other way is a permanent stop at say 12 " and then make pieces 2,3, or what you need to get proper location.
There are safe lights for color.
Rotary cutters are very nice, but they tend to have a plastic pressure strip next to the cut. Color paper unprocessed is sensitive to scratches and the plastic will scratch. You will not see them until the print is finished.
It is true that scratches can occur but there are a couple caveats.
One is cleanliness, the other is that the emulsion has been on the outside of all the rolls I've gotten (but that is not a given), in my cutting process that means emulsion down (against the base board, not the plastic) as it comes through the cutter.
I get a scratch in about one of 50-100 sheets, so pretty rare.