For regular glads anything is fine even 40 size... eventually you will get him to retirement.
For premium slaves, get one with at least 1200 with all stats over 89, except you're going for some extreme build. You can get away with extreme builds, but thats more for experienced players.

With overall good stats you can change your skills/builds without sucking imagine a war glad... so many builds with many different stats needed for each one.