It's generally heat fluctuation that causes vinyl to shrink, any calendered vinyl regardless of quality will shrink to some degree (whether you notice it or not).
Unless it's the cheapest of crap or removable stuff then it's unlikely to shrink any more than about 1-2mm. If shrinking is a cause of significant concern then look for vinyl that is cast instead of calendered.
I'm not saying don't take Steve's recommendation, as it would save you a bit of money, and the seller can actually spell "adhesive"
FYI any quality sign writing vinyl will be significantly thinner than the OEM stuff that's on there.