How big does the code actually need to be?
Print QR codes usually get scanned from arm's length, but a yard sign gets scanned from a car. Someone parked at the curb, say 3-5 meters (10-15 feet) away, needs a code that's genuinely large to read comfortably through a windshield or car window — think 25-30cm across or more, not the postcard-sized code that works fine on an indoor flyer. It's bigger than most people expect the first time they size one for a sign.
If the sign is meant to be walked up to — an open-house sandwich board by the front door, say — you can size it smaller, closer to normal print rules. But for anything facing the street, oversize it. A slightly-too-big code costs nothing; a too-small one just gets driven past and ignored.
Weatherproofing your printed code
A sign that sits outside for weeks needs material that can take it — laminate over a paper print, or a weather-resistant substrate like coroplast or vinyl made for yard signs. Untreated paper wicks moisture and the ink can run in a downpour; UV exposure fades ink over time too, and faded ink means lower contrast between the modules and the background, which is exactly what a scanner needs to tell them apart.
Stick to a high-contrast combination — dark modules on a light background print reliably and stay readable even as the material weathers a little. Keep a clean quiet-zone margin around the code, and for a listing that sits on the market a long time, it's worth swinging by after a hard storm to check the sign is still holding up.
What to link the code to
Link to the specific listing page — photos, price, square footage, the details for this exact property — rather than your agency's general homepage. Someone scanning from the curb already knows they're interested in this house right now; a homepage makes them dig for the thing they just stopped to look at, and you lose most of that curiosity in the click.
A lockbox code can point to the same listing page, though it's usually scanned up close by an agent rather than from a car, so it doesn't need the oversized street treatment. Since it's a static code, the same link can run on the yard sign, the lockbox and a flyer box with no extra setup — just make sure it's the right listing before you print.