Export it as a PDF (you can't scale while exporting). Place the PDF into another InDesign file. Scale the PDF to the size you want, then export a second PDF file.
If you're sending this to a printers to be professionally printed they can do the scaling for you.
But I'm with Steve but with a minor change.
Instead of placing a PDF into InDesign document -
You can place the actual InDesign file into another file and scale it that way then export to PDF.
You can use Multipage Importer Script to make it less painful http://indesignsecrets.com/zanelli-releases-multipageimporter-for-importing-both-pdf-and-i ndd-files.php
There's a small chance of text reflow when placing one ID file into another and scaling because the text remains "live" even though you can't edit it. I've seen thi shappen, and apparently it's caused by kerning in the font being active only for sizes over a certain threshold.