Which RSA key size should I choose?
2048 bits is enough for most purposes and such a key is generated faster. 4096 bits is chosen when you want a margin of safety for years ahead. Keys shorter than 2048 bits are no longer safe and remain in the generator only for compatibility with legacy systems.
Does Randogram store the private key?
No. The key pair is created on the server from a cryptographically secure source of randomness, returned in the response and never stored anywhere. The database keeps only the selected key size, for statistics.
How do I use the key to connect over SSH?
The private key is returned in PEM format: save it to ~/.ssh/id_rsa and restrict the permissions with chmod 600 ~/.ssh/id_rsa. The public key is returned in OpenSSH format — add it as a single line to ~/.ssh/authorized_keys on the server.