I'd say left-leaning sources are those that tend to report negatively on conservative politicians and positively on liberals. The Harvard study purports to measure what constitutes negative coverage, so this seems like it could be measured.
About numbers, it seems today often Fox reports favorably for the GOP, while CNN, NBC, MSNBC, NPR, and the BBC seem to favor the political left. (I think the Harvard study seems to suggest this.)
I suppose a microcosm of the 'balanced list' I'm describing might be 'Real Clear Politics'. This site posts leftist articles, then immediately follows them with right-leaning articles in equal numbers one after the other. It's an interesting way to get news, and it allows the reader to study different biases, often on the same topic.