I've got a hack set-up I think some other people here might be interested in. Using Firefox's built-in PostScript printer support -- which allows individual printers to have their own margin etc settings and a custom command run on the output -- I can "print" from Firefox and have the result droped onto my Reader as an appropriately-sized PDF. The process is currently Unix-y only, but someone sufficiently determined could probably produce one which works under Windows too.
I describe what I've done in more detail and provide scripts & settings in a post on my blog:
Hope people find it useful!