The largest balloon is a sounding balloon. Sounding balloons are the workhorse for weather forecasting. Typically, sounding balloons will carry a small electronic package (radiosonde) which transmits the data necessary for weather prediction, pressure, temperature and humidity. Some radiosondes also contain a GPS receiver/transmitter. As the balloon ascends it changes position. Position data is transmitted to a ground station where the changes are converted to wind speed and wind direction.
Depending on observation requirements sounding balloons vary from a burst diameter of 7 feet at an altitude of 70,000 feet to a diameter of 43 feet at an altitude of 125,000 feet. Burst altitudes will change due to increased payloads, weather conditions at the tropopause and other environmental factors.