The answer is that the size of a snowflake is determined by the temperature in the air. Generally, the colder the air aloft is, the larger and more elaborate the snowflake will be in shape.
Snowflakes form when water vapor condenses onto particles such as dust or pollen in a cloud. When they are cold enough (below freezing), water molecules in the vapor will freeze to create ice crystals.
When the air gets warm enough, the ice crystals will begin to melt. This makes it easier for the ice crystals to stick together and form bigger flakes.
Sometimes the flakes will form into different shapes, such as needles, dendrites, plates or columns. These can be very pretty to look at.
These different types of snowflakes are influenced by various factors such as wind direction and weather conditions. These factors can change a snowflake several times before it hits the ground.
It's also important to understand that the amount of snow a snowflake accumulates depends on the temperature and humidity in the air. Drier, lower-altitude conditions produce smaller flakes, while warmer, higher-altitude conditions allow for more accumulations.
There are two main reasons why snowflakes fall slowly through the air: They have a light weight and large surface area, which slows them down as they go up into the sky. They are also often caught in updrafts, which can halt or slow their descent.