[CentOS] SSH Question relating to Public and Private Keys

Hi People,

The Linux Environment I am responsible for is using ssh key pairs to allow access to a number or accounts on a number Linux Servers. I currently have the opportunity to re-design some of this. So I would like to tap into peoples experiences to see what might be some good changes to make. Specifically I have a couple of questions

1. Currently all of the key pairs we are using have empty passphrases is it worth the effort of changing this and setting up ssh-agent compared to what you gain in security by doing this ?

2. At this stage I am going to use RSA Keys of the default size, is this generally the best approach?

Thanks for any thoughts, and have a nice day :)