DSA is used by default. When RSA is used, the default RSA key size is 2048.
The following example shows the configuration of the SSH Server Key type to RSA 4096
SLX(config)# ssh server key rsa ? Possible completions: [2048] 1024 1024 bits RSA key 2048 2048 bits RSA key [default] 4096 4096 bits RSA key SLX (config)# ssh server key rsa 4096
When the SSH Server key is changed, restart the SSH server from EXEC mode using the ssh-server restart command for the new configuration to take effect.