From the command line:
# ssh-keygen -t rsa -b 4096 -o -a 100
When saving a new-format private key (i.e. an
ed25519 key or any SSH protocol 2 key when the
-o flag is set), this option specifies the number
of KDF (key derivation function) rounds used.
Higher numbers result in slower passphrase
verification and increased resistance to brute-force
password cracking (should the keys be stolen).
Specifies the number of bits in the key to create.
For RSA keys, the minimum size is 1024 bits and the
default is 2048 bits. Generally, 2048 bits is considered
sufficient. DSA keys must be exactly 1024 bits as
specified by FIPS 186-2.
-o Causes ssh-keygen to save private keys
using the new OpenSSH format rather than the
more compatible PEM format. The new format
has increased resistance to brute-force
password cracking but is not supported by
versions of OpenSSH prior to 6.5. Ed25519
keys always use the new private key format.
-t dsa | ecdsa | ed25519 | rsa | rsa1
Specifies the type of key to create. The possible
values are “rsa1” for protocol version 1 and “dsa”,
“ecdsa”, “ed25519”, or “rsa” for protocol version 2.