The quote in question is actually the first description of Ichabod Crane as a person. Irving's intention was to point out his peculiar physical features as well as his somewhat striking physical appearance, which was in contrast to his occasional gentleness and sensitivity but not in contrast to his strangeness or his personality in general.
The cognomen of Crane was not inapplicable to his person. He was tall, but exceedingly lank, with narrow shoulders, long arms and legs, hands that dangled a mile out of his sleeves, feet that might have served for shovels, and his whole frame most loosely hung together. His head was small, and flat at top, with huge ears, large green glassy eyes, and a long snipe nose, so that it looked like a weather-cock perched upon his spindle neck to tell which way the wind blew. To see him striding along the profile of a hill on a windy day, with his clothes bagging and fluttering about him, one might have mistaken him for the genius of famine descending upon the earth, or some scarecrow eloped from a cornfield.
Irving basically presents Crane's physical appearance as a reflection of his character. He describes him as tall and lanky, similar to a crane; unattractive, but strange and awkward enough to stand out in the community. His head is small and flat, which might symbolize his inability to think rationally or to think about anyone's else interests aside from his own.
His eyes are described as glassy green, which might indicate his inability to see clearly (both literally and metaphorically) past his own greed, which he consciously or unconsciously hides behind his gentlemanly behavior and agreeable nature. His clothes hang loosely on his tall and skinny figure, as if he's a scarecrow or the personification of famine, which is an interesting description, as Crane is actually someone who enjoys food.
Irving uses these metaphors and comparisons to show how Crane stands out in the community, both with his physical appearance and his strange personality, and how he attracts attention, despite his apparent unattractiveness.