I'm going to agree with both CodeInChaos and Brad here. They make some valid points about the questionable approach you're taking to making ssh secure.
Be that as it may, you could look into PAM. If you inspect /etc/pam.d/sshd, you can see which security modules sshd uses. You could write your own module to ensure that the public RSA key used for authentication was generated using 8196 bits, and then place the module in /lib/security. Then add a line into /etc/pam.d/sshd that looks like this:
account required /lib/security/<my_fancy_security>.so
I'll admit that I'm not sure what mechanism you would use to check the key used. For example, you could try checking the character length of the public key being used. On my machine, generating a public-private rsa pair of keys always produces a public key 1424 characters in length and a private key of 6446 characters in length. Simlarly, generating an rsa key with 2048 bits always produces a public key with 400 characters and a private key with 1766 characters. Note that this seems to be OS-dependent; my laptop produces keys of different lengths. That's not an insignificant limitation.
That's the best I can think of right off. If sshd doesn't provide a solution for you, you might need to get creative and start coding.