I often do larger signs in Inkscape.
I usually design so it fits on one page, save a copy as PDF. (Keep in mind the percentage you will ultimately enlarge)
Open in Adobe Acrobat Reader and use the poster feature and then enlarge at the percentage that will give me the size I want. Steve gives instructions on doing this on his blog.
Also, you can change the page size in Inkscape (Document Properties >> custom size). Save as PDF and Acrobat Reader in poster mode will split the pages and add the cut marks.