I have a keypair. How do I determine the key length?

Solution 1:

openssl rsa -in private.key -text -noout

The top line of the output will display the key size.

For example:

Private-Key: (2048 bit)

To view the key size from a certificate:

$ openssl x509 -in public.pem -text -noout | grep "RSA Public Key"
RSA Public Key: (2048 bit)

The first (2048) is the bit length of the key:

 $ ssh-keygen -lf /etc/ssh/rsa_key.pub 
 2048 91:1c:ae:17:16:...