Answer: I believe the main idea is Ichabod's appearance and how weird or unusual he looked, the paragraph is obviously focused on how he looked. As for the supporting details I would personally say that it would be: how he rode, for example "he rode with short stirrups, which brought out his knees nearly up to the pommel of his saddle." His clothes "A small wool hat rested on the top of his nose, for so his scanty strip of forehead might be called, and the skirts of his black coat fluttered out almost to the horses tail." Then the last supporting idea is how his appearance was really not normal "and it was altogether such an apparition as is seldom to be met with in broad daylight."
Explanation: In conclusion the reason why I say the main idea is how he looked is because of the last sentence in a way. From the way he was described-including his dressing- I thought he was moving around in the night or something, and how it was based on how it looks.
I hope this helps. Have a nice day!