How to measure length of project

When instructed to knit until project measures 17 inches, do I stretch the project or do I just lay it flat and measure it like that? On some garments, it seems self-explanatory, but on a pattern I’m using currently (which I’m knitting for someone out of town, so I can’t measure it to her) it seems as if this garment will be way too long. Any help here?

Don’t stretch it, usually you would lay it down on a table and measure.