To check the key length of a certificate, use the openssl command.
Here is how to check an RSA private key length:
$ openssl rsa -text -noout -in private.key
The top of the output will have:
Private-Key: (1024 bit)
For a certificates (.crt or .pem), use this command:
$ openssl x509 -text -noout -in certificate.crt
.p12 certs:
$ openssl pkcs12 -info -in keyStore.p12
The output will have the key length in a dump of all other certificate info:
RSA Public Key: (2048 bit)
