From The weather Vane Wiki
Hail are large ice pellets that form in the updrafts of a thunderstorm. In the cold air in a thunderstorm these ice chunks freeze and when they get tossed up into the thunderstorm over and over again by updrafts more and more layers form until the hail gets heavy enough and falls down to earth. The temperature does not have to be cold out the lower levels for hail to form and fall. Hail is always much bigger than sleet.
Hail to 7.00" feel in Aroura Nebraska on June 30, 2003. The hail was on the ground for several minutes (mabey over 10 minutes) before being measured and could have been larger. Hail to around 7.00" also feel in South Dakota in the south east part of the state during August of 2007. I forget the exact day but this hail left craters in the ground and several houses and cars recieved damage. This hail was on the ground for 10-20 minutes before being measured having that said it was not as large as the hail that fell at Aroura Nebraska I believe. Still this breaks South Dakota's old record of 6.00". A hail storm near Seminole Texas I think in the 90's dropped golfball sized hail for nearly a hour.
HAIL SIZE (in.) OBJECT ANALOG REPORTED .50 Marble, moth ball .75 Penny .88 Nickel 1.00 Quarter 1.25 Half dollar 1.50 Walnut, ping pong 1.75 Golf ball 2.00 Hen egg 2.50 Tennis ball 2.75 Baseball 3.00 Tea cup 4.00 Grapefruit 4.50 Softball 5.25 ? 6.00 ? 7.00 Volleyball sized hail.