Even without the key they start to look pretty dense. You can definitely fit one in a QR code, they just start to become less reliable to scan (especially on cheap devices), and they go from looking nice to looking quite ugly. Technically most X.509 certs would have been within the limitations of QR codes (though I don’t think there is an upper bound to how large they can get), but I realized it just wasn’t fit for purpose and moved on to something else.
Are you using 8 bit encoding? An alphanumeric mode QR code containing base 64 encoded data provides less capacity. In binary mode even a 4096 bit RSA secret key fits while ECC keys produce smaller codes.
The qrencode tool has an 8 bit mode but not all decoders can handle binary data. For example, my phone shows me mangled results and I can't redirect them to a file. Like structured append, it doesn't seem to have much support.
