as you’ve discovered, you need to tell the printer what size of piece of paper you have in the machine, doesn’t matter what size of print you want on that piece of paper. In this case it’s a 24” wide roll which means your paper width is 24”.
I told LR I was using 24" roll paper. In LR Page Setup I have many "Page Size" definitions, most of which are less than 24" wide (8-1/2" x 11", etc.). I had a custom page size of 20" x 60" and had selected "borderless printing". LR then showed me a cell width and image width of 20", but it printed 24" wide. If I deselect "borderless printing", it won't allow me to select 0" borders and shows a cell width and image width of 19.76". Following Dan's suggestion I then resized my page to be 24" x 62" and zoomed the cell size to be 20" wide. What I've learned from this is that I need to think things through because LR does not always do so.
Some anal details I've discovered: When you use the Canon driver to define a custom page of any size, it creates a page that size AND ANOTHER page of the same size that is borderless. In my example I defined a page size of 20" wide by 60" high and named it "Custom - 20x60". This action created two page size options in the Canon driver, one called "Custom - 20x60" and one called "Custom - 20x60 borderless". I selected the second option when I printed. I've just discovered that if I select the first (non-borderless) option and then select the "borderless" box in the Canon driver to force it borderless, I get a warning message from Canon that says, "Scales the document to fit the roll paper width...". However, this warning message does not appear when I select the "Custom - 20x60 borderless" driver that was automatically created by the Canon driver.
Thanks for everyone's help.