Spotters should report hail when it is 1/2" diameter or larger.  If the hail varies in size, report the largest hailstone.  Hail covering the ground or accumulating should be reported to the NWS, even if the hailstones are smaller than 1/2" in diameter. 

It is sometimes easier to relate hailstones to objects than to give an approximate measurement.  However, if you choose to do this, please avoid ambiguous object sizes such as marbles or eggs.  Some of the more common hail sizes are as follows:
 

Hail Diameter

Description
1/4"  Pea Size
  3/4" (Severe Criteria)    Penny Size
7/8" Nickel Size
1" Quarter Size
1 1/4" Half Dollar Size
1 1/2" Ping Pong Ball Size
1 3/4" Golfball Size
2 1/2" Tennis Ball Size
2 3/4" Baseball Size
3" Teacup Size
4" Grapefruit Size
4 1/2" Softball Size