Pretty much anything text-based. Website links are the most common, but you can also encode plain text, your home Wi-Fi details so friends can just scan to connect, or a digital business card.

If you're putting it on a business card or a flyer, a standard 300x300 pixel size works great. If you're printing a massive poster, bump up the size so people can scan it from further away.

Never. These are static codes, meaning the data is hard-baked into the image itself. As long as the link you put in it doesn't break, the code will work forever.