Krakatoa 1883 (VEI 6) was merely 10 x Mt St. Helens 1980 (VEI 5). Tambora 1815 (VEI 7) was 10 x Krakatoa and produced a year without a summer ejecting 150 cubic kilometers of rock, 200 million tons of aerosols.
Pinatubo 1991 (VEI 6) ejected 5 cubic kilometres of rock and 15-30 million tons of sulfur dioxide, the average temperature in the Northern Hemisphere was reduced 0.5 to 0.6C and the entire planet was cooled 0.4 to 0.5C. The maximum reduction in global temperature occurred in August • 1992 with a reduction of 0.73.
A Yellowstone VEI of 9 (or super eruption) is 300 x Krakatoa, and that is only if it comes in at the low range and short duration of possible super eruptions.
It could just go boom like St Helens and be done or, more likely, erupt for days, weeks, or months - depending how long it takes for the magma chamber to empty. Remember there is something like 400 miles x area of stuff to come out. Also it depends on what the compositon of the stuff will be - certainly a lot of sulfer dioxide, other aerosols, ash, and rock. Not to be fogotten is the last super volcano eruption in 74,000 BC of Mt Toba which eliminated most of the human race. And of which, we are the progeny of the handful of survivors.
The entire world could be plunged into years without summers, springs or falls. Some say that a super eruption during even a slight cooling trend and with an extreme low in sunspot activity (both of which we are in now) could very well trigger not just a new Ice Age but a Snowball Earth (http://en.wikipedia.org/wiki/Snowball_Earth) effect.
VEI=volcanic eruption index.