Would you be able to use something like Visio or Dia (http://dia-installer.de/index.html.en)?

You could also use Illustrator or even Photoshop (or gimp) quite easily - treating the image as a giant whiteboard (very easy if you have a tablet). In Photoshop you can open massive images - 30000x15000 is the largest I usually use because I can still save it out as a bmp for use elsewhere.