What is the largest earthquake attributable solely to volcanic activity
Earthquakes associated with volcanism or volcanic rifting are typically small. They are caused by over-pressurized magma as it moves around and intrudes into brittle rocks.

Sometimes, e.g., in Hawaii, you get earthquakes due to the gravitational sliding (e.g., the M6.9 earthquake in 2018 beneath Kilauea) as new eruption causes the volcano to grow in size (adding mass) which in turn cause it to slide over the older volcanic rocks underneath. 

I don't think there's an exact upper bound for such an event but it is unlikely to exceed ~7.5. Another thing to keep in mind is that rocks near active volcanoes tend to be relatively hotter which in theory should cause less shaking during the earthquake (due to lower stress drop).