Should I list talks on my CV given by co-authors? On my CV, I have a few talks listed where I was not the presenting author (though I am a co-author on the work).

I have left them on my CV because as a graduate student, my CV is pretty slim. However the work in question was recently published in a journal, which now appears on my CV.

Should I delete all the talks where I was not presenting author? I'm not sure adding length to my CV is really appropriate since the information is fairly redundant (all the same line of research and similar talk titles, just different conferences).
The simple answer is no. You should not list items that you have not performed. In this case it would be listed under "talks" (equivalent) and that would be wrong. On th eother hand it represents work that you have done and as such it can be listed but probably under a different heading. The important thing is that it represents what has actually happened. I can see a heading called, for example, "Other work presented at conferences" and then list the talk and state that you was part of the work but that it was presented by someone else. Again, the importance lies in formatting it so that the actual conditions are very clear.

When the CV is short, I think such additions may make sense, later in a career these posts will lose their importance relative to other headings.