[CentOS] can't change login screen resolution

After boot up, on the login screen, I can't seem to find how to change the
resolution.  I've have it set at one resolution after login for all users,
but still, the login screen remains the same.  It's a bit annoying because
I get a warning message from my monitor of incorrect settings and
sometimes it wacks out.  I think the refresh rate is what is causing it. 
Thanks.