Yeah, I'm pretty sure you can't set it up to auto-adjust the page size to fit everything on one page. A further frustration, though, is that you cannot manually exceed 36" (at least I can't on my machine). I've taken a look at the various DLLs related to the Journal printer and I cannot find any tweakable parameter, so either the 36" limit is hard coded in the driver or it's a Windows imposition. I couldn't find any way to get at the latter.
Yet another reason why Microsoft should make their Journal software on Open Source.