Abstract: Based on the observation of lightning mapping array, the statistical distribution of characteristic parameters describing the lightning flash initiation and size in a supercell storm occurring in New Mexico, United States on 5 October 2004 is studied. A method automatically identifying the start and end of the negative leaders in initial stage (IS) of lightning is developed. And the flash convex hull, total channel length, horizontal and vertical extent are used to represent the scale characteristics. Distributions and characteristics of flash initiation and size in this storm are shown as follows.Median values of the duration, three-dimensional displacement, vertical displacement and the average displacement velocity for the upward (downward) negative leaders during IS are 13.5 ms (7.5 ms), 1.4 km (1.0 km), 0.9 km (0.5 km), and 9.2×10⁴ m·s^-1 (1.2×10⁵ m·s^-1), respectively. In addition, the average flash initiation velocity decreases with height from 6 km to 11 km. With time going on, the speed of upward negative leader in initial stage decreases before 24 ms (to ensure the samples is larger than 100), while that of the downward negative leader increases before 12 ms (to ensure the samples is larger than 50). Moreover, negative leaders are dominantly tilted in initial stage, considering that the median angles between the 3-D displacement direction and the vertical direction are 40° for upward leader and 54° for downward leader, respectively.The probability density distribution of flash size described by flash convex hull and total channel length can be well fitted by negative power function, also showing that the distribution and evolution of flash convex hull is consist with that of total channel length. The median of flash duration is 271.0 ms, and the mean of that is 329.1 ms. The flash duration time and size are not significantly correlated. The flash with long duration time is not necessarily large. Moreover, the median of flash horizontal extent is 6.1 km while the vertical extent is 4.3 km, and there are 83% of flashes whose horizontal extent is greater than vertical extent. Flashes with horizontal extent greater than vertical extent are mainly initiated at 8.5 km high, and those with vertical extent greater than horizontal extent are mainly initiated at 11 km high. Greater horizontal displacement of the leader during initial stage accompanies less vertical extent, which indicates that the leader displacement direction at initial stage has an important influence on flash vertical scale.