To generate ssh key with ed25519
use:
$ ssh-keygen -t ed25519 -C "<email>"
If you want to use rsa
for reasons like legacy system, then do it with at least 4096 bits:
$ ssh-keygen -t rsa -b 4096 -C "<email>"
rsa
concidered less secure and ed25519
keys are shorter for same level of security, which makes them faster.