Probably about 1/10 of a Yellowstone. This ones last one was Tambora sized, equal in discharge to about 70 Mt St. Helens...
Will need vast expansion of heated greenhouse for 2 to 5 years. Expect loss of production of typical grains. Grow short season varieties like buckwheat if sufficient conditions. So goes meat production with loss of grain supplement to any available grass. Raise goats, pigs, fish. Orchards may survive but not be productive. Dim sunlight affects all aspects of farming / ranching.
May take extended time for atmospheric currents to transport dust and aerosols below the equator into that hemisphere, which might allow agriculture and so on to fare a bit better. Concentrate some effort toward expansion of production in friendly land regions—Australia—South America.
Forget air travel for a long duration—months to years for ash to be removed by cloud droplets. Perhaps advanced dust separation could be devised for turbine engines at a performance penalty. This ash problem would also be shutting down natural gas fired electrical generation turbines as well—any turbine engines at all will suffer.
Coal fired boilers and nuclear will have a resurgence! Solar will ineffective—dim light and dust fouling. Wind???