Labels on sheets other than 8.5 x 11

I use the label setup a lot, as a large part of my business is digitally printing die cut (or more precisely, kiss-cut) labels. I have more than 70 user defined layouts. Printing digitally, of course, it is always more cost effective to print on larger sheets, so I want to start printing labels on larger sheets (11x17 or 12x18) as the click charge is the same regardless of the sheet size.

My problem is, it seems the labels can only be designed on 8.5 x 11 sheets. I cannot find a way to setup a larger sheet. Does anyone know of a way to change the sheet size on user-defined labels? Is there perhaps a macro that can work around this apparent limitation?

I realize I can achieve some of the same effect with imposition layouts, but I cannot fine tune positioning this way to ensure my print lines up with the pre-cut label sheets.

Any advice or help would be greatly appreciated.