I have a need to print 12″ x 30″ through my HP Officejet 7000 Wide Format printer. Unfortunately, the ability to choose a custom size (outside of the predefined sizes) is not available. It appears that HP removed the ability to make custom sizes a few years ago (when I originally purchased the printer), but they reintroduced the feature in later drivers.
The solution is to change out the printer driver to a more recent HP version – even if it is not specifically the same printer. Doing so can display a GUI where the dimensions can be changed. I found this out by finding this very stark link:
On a side note, it is unfortunate the printer will not print edge to edge if the width is over 11.5 inches.