"Size means the size of the avalanche. Small avalanches are D1 in size or not quite large enough to bury a person unless other factors like terrain traps contribute to magnify the consequences of the avalanche. Large avalanches are D2 avalanches meaning that they can bury or injury people. Very large avalanches are D3 avalanches that can bury and destroy a car, damage a truck, destroy a small building or break a few trees. Historic avalanches are D4 and D5 avalanches that can destroy a railway car, large truck, several buildings, a small village, or a forest area up to 40 hectares. "
Source:https://www.sierraavalanchecenter.org/how-to-read-avalanche-advisory#:~:text=Size%20means%20the%20size%20of%20the%20avalanche.&text=Historic%20avalanches%20are%20D4%20and,area%20up%20to%2040%20hectares.