Hi Jason,
I've had a look at the Search Metrics 'Ranking Factor Study' PDF and have looked at the relevant information. I believe the 'negative correlation' you mentioned refers to the fact that the H1 is now less important regarding the ranking and the SERP, according to their report. The H1 tag is roughly 55 - 60 characters, or 4 - 5 words as Moosa Hemani has said.
I hope this has helped you understand what it means!
