I hope I understand. I have a 9890 driver here and that should be close enough..... it handles 44" wide rolls.
I assume the 9900 does too.
Custom width = what?
I set for 18.0 inches and it likes it.
See attached.
If I got the question wrong, please advise.

Fred
PS I just installed the 9900 driver, and it is the same operation as the 9890.

Rani,
The auto cut box is asking what size roll so it knows how big of a cut to the roll when the print is finished.
Turn off auto rotate and leave 44 inches for cutting.
Turn of auto cut too if you do not have to do it.
Just tell Custom size that your page size will be 18" x 30" (or whatever)
Here's a shot of the Print Preview screen.  
All is well. Also a screen shot of the print in the queue showing Print to Page at 17.766 x 29.766, the printable area of the 18 x 30 inch user defined page.