I've got a design that needs to be printed for an 4' by 8' window decal. The design is entirely vector, so I know that it can be resized, but what I don't know is how I should save it. Should I just give the decal printers an .ai file with say a 400x800px artboard, or should it be prepared some other way?
If you can be assured that the decal cutters can take an AI file, you are ready to go. Ensure that you don't have a newer version of Illustrator than the vendor. If you do, use File, Save As and select a version to match your vendor. It would not hurt to create a PDF file at the same time, perhaps an EPS just to cover all the bases. Some vinyl cutters are using CorelDraw. If your version of Illustrator supports .CDR export, drop one of those in there too. It's not that much more work to create four or five different format files.
You are correct that being vector means easy enlargement, as long as your vendor knows the final size.