You have my sympathies. It is probable that you will not get MS Windows Gimp to print 8.5" x 17" even when your printer spec says up-to 47" long.
The same problem has been reported several times over the years, example: https://gitlab.gnome.org/GNOME/gimp/-/work_items/8250
The best solution is do not use Windows Gimp for printing, use some other application.
You have a User Defined size set in your printer driver, but Gimp ignores it.
This using XnViewMP https://www.xnview.com/en/xnview-mp/ It is free My Brother printer has a maximum length 14" and it looks like that will print. Or, you could try something like LibreOffice.
Edit: Another way is export to a PDF then use a PDF viewer to print. Only caveate. I use linux and not sure on best Windows PDF viewers.
The same problem has been reported several times over the years, example: https://gitlab.gnome.org/GNOME/gimp/-/work_items/8250
The best solution is do not use Windows Gimp for printing, use some other application.
You have a User Defined size set in your printer driver, but Gimp ignores it.
This using XnViewMP https://www.xnview.com/en/xnview-mp/ It is free My Brother printer has a maximum length 14" and it looks like that will print. Or, you could try something like LibreOffice.
Edit: Another way is export to a PDF then use a PDF viewer to print. Only caveate. I use linux and not sure on best Windows PDF viewers.