Can somebody please help me with this question?

Can somebody please help me with this question? its due soon and I don't know how to do it!

The annual Darwin Award is given to the person who did the gene-pool the biggest service by killing himself/herself in the most extraordinary way. The 1997 Award went to Larry Water of Los Angeles (who survived his award-winning accomplishment). He purchased 45 weather balloons and several tanks of helium, inflated the balloons, attached them to a lawn chair and tied himself to the chair. When he cut the cord attaching the chair to his jeep he shot up into the air and didn't stop climbing until about 11,000 feet. He was rescued by a helicopter but immediately arrested for violating Los Angeles International Airports's airspace.

Question:

make a rough estimate of the total volume of helium gas needed to cause such an incident. If all that helium were used to fill one single spherical balloon what would be the approximate diameter of that balloon?