For a printed image, yes, but for a blank sheet of papers is useful to dumb down and think as most drivers do: The height of the blank paper is always the longer dimension (if square it doesn't matter). So, if you need to print a 18x24" image on a 24" roll paper, you should set the custom size like this: Height 24", Width 18", Orientation Landscape.
If you swap the dimensions (and the driver allows it as Canon drivers do) the paper will be horizontal while in "portrait" orientation, so it is very easy to mess things up.
To me it's easier to think of it in terms of the printer and of the image separately. The width of the paper is always
the width of the actual paper as it is placed in the printer. If you have a 24" roll loaded, then the width for any created size should be 24". the height is either the actual height if a sheet, or the desired cut length if a roll.
Landscape and portrait orientation are instructions to the driver on how to orient the image on the paper that is being printed. So if I wanted a 18x24" print on 24" roll paper, my settings in a custom paper size would be width of 24", height of 18". If my image itself was a landscape image, that orientation is the same as the paper, so I wouldn't select landscape, but leave it at portrait. If I were choosing one of Epsons built in preset sizes, I would choose from a 24" width option.
Paper sizes are created without regard to the final image that is being printed, just simply the size of the desired final print, with the width being the actual paper width as it is inserted in the printer. Landscape and portrait are selected based on the orientation of the print as it relates to that piece of paper. This is the confusing part because if you create a paper size that is wider than it is tall, and you print a landscape oriented image on it, the actual correct setting is portrait, because the image is in a portrait orientation to the piece of paper that is being printed.
If you transpose any of these items you can get some very strange results, such as nothing or only a small piece of the image being printed.