FYI, based on Miguel's research, here's a modified version of the StreetIndexReport.py that will print in 11x17" size when in landscape mode. One thing that tripped me up for a while was that pdfs measure from the bottom left, not top like I assumed. So if you need to modify settings, keep that in mind.
Also Miguel, I think you could modify pagesizes.py to add your custom size. Throw it under line 25:
LETTER = (8.5*inch, 11*inch)
LEGAL = (8.5*inch, 14*inch)
ELEVENSEVENTEEN = (11*inch, 17*inch)
MYCUSTOMSIZE = (11.75*inch, 15.75*inch)
Then replace letter with MYCUSTOMSIZE in the script for the tool. Not tested, but that seems like it should work. Then play with page settings to place your elements. It might help to print a sample page and then use a ruler to figure out where you want them.