QR codes or 2D barcodes with URL links are absolutely possible on custom printed shields!
I printed 5 different sizes so you could a get an idea how much room they take up.
With regards to minimum size I was able to get the 1/2″ size to scan but it had to be inches away from my phone and the camera had trouble focusing. While every device is different I still wouldn’t recommend the 1/2″ size. The 3/4″ size seemed to have absolutely no problems scanning and that’s what I’d recommend.
If you’d like to get that actual image file that’s below, you can access it here.