The structure of this poem mimics the movement of a winter day.
The first stanza mimics the movement of a snowflake through the air. The snowflake itself is debating if it will go across a barn or through a rut. It's not sure where it wants to go. Furthermore, snowflakes do not travel straight down; instead, they are pushed and pulled by the wind. As a result, the first line of the stanza is long, while others are shorter. This mimics the back-and-forth motion made by the snowflake.
Similarly, the second stanza mimics the "narrow wind." The length and sound of the lines mimics the howling nature of the wind, which "complains all day."