On May 18, 1980, Mount Saint Helens made American history with an eruption that took 57 human lives and killed thousands of animals. It has released only a quarter of a cubic mile of ash, but it is the influence of the prevailing winds that allows us to measure the area and thickness of the ash cloud. The prevailing winds of 1980 made the Mount Saint Helens ash cloud a bit like this:
The reason I brought it up is that it inspired this particular alternate scenario. In an alternate Earth, there is a volcanic island six miles wide and 8,157 feet above sea level. It is located 42°54′59″N but 50 miles west of the coast. Let us say that this fictional volcanic island, which I'd christened "Wizard Island", was the alternate volcano that erupted in 1980, not Mount Saint Helens. It's not just the different location of the eruption that would affect the outcome of the question, but also the different volume of the eruption. Wizard Island erupted a dozen cubic miles of ash, 48 times bigger than Mount Saint Helens in OTL. Using the provided information, how big and how thick would the resulting ash cloud be?