Couple of ways…
If it’s a one off… in the top right hand corner of the Onyx RIP Queue you’ll see the printer info (current media, page size, placement, etc…) If you click on the little cog symbol then click "custom" in page size, you can enter a custom width (just enter the length as something ridiculous like 999m).
If it’s something you feel you want to save for future use, then along the top of the screen of Onyx RIP Queue, if you click on "configure printer" (all other open Onyx applications will shut), then click on "page sizes" tab along the top of that menu, then click "add" on the right hand side. Select "custom", enter the width and whether it’s a roll or sheet and select inches or mm. Before pressing "OK", I always name the new size whatever the width is, then it’ll be nice and easy to find later on.
Click "OK" until the menu’s disappear and you’re back in Onyx RIP Queue. Then you’re new width should be selectable in the top right hand corner menu by clicking on the cog and selecting it!
Let me know if neither of those work…