Question on Roll Printing

Hi all,

Need your help debugging my thinking.

I want to print on 17" wide roll paper (using an Epson SureColor
P5000). I want the minimum dimension to be 17", so the length will be
about 25" with the image's aspect ratio.

However, in Gimp using the 
Gutenprint options, the long dimension is fixed at 17" and greyed out;
so the print will be 11"x17".

How do I direct Gimp and Gutenprint to print a "long landscape"? What
setting am I overlooking?

Thanks,

Erik