Using the kinetic molecular theory, what effect does temp and pressure have on the size of a weather balloon?

Weather ballons filled with helium carry instruments into the upper atmosphere to measure weather conditions. At 10 - 15 km above sea level, the temperature is very cold and the atmospheric pressure is very low. Using the Kinetic Molecular theory, what effect does these factors have on the size of a weather balloon that is released at Trenton Air Station.