The אבנט is one of the lesser know of the בגדי כהונה.
Length - It doesn't give a length in the Chumash. The Yerushalmi (Yoma 8:3) states that it was 32 אמה long. An אמה is between 18 and 24 inches making the אבנט a whopping 48-64 feet long.
Where was it worn on the body? - I always thought that the אבנט was a belt. However, the Gemara in זבחים י"ט learns out from a pasuk in נביא that the Kohen wore the אבנט higher then a belt. The gemara states that he wore it right below the heart where the elbows meet the body (if you hold your elbows against your body).
How many times did the Kohen wrap it around? - As we saw above the אבנט was very long. The Shita Mekubetzes (Erchin 16a) states that he wrapped it around 32 times כנגד לב. However this is very difficult. The average waist/chest is at least 36 inches (3 feet). 64 / 3 = 21.3 and therefore I don't see how it would be possible to wrap it 32 times.
What is the אבנט מכפר for? - The Gemara in Erchin (16a) comments that just like Korbanos are מכפר the בגדי כהונה are מכפר. The Gemara explains that the אבנט is מכפר for הרהור הלב (meaning avoda zara where you are חייב for thought). The Yerushalmi has another interpretation that it is מכפר on someone who is not straight. The reason being that the אבנט was not wrapped straight but rather at an angle so that it didn't make such a big lump in the front and back.