How is the position of a peak on an IR spectrum determined?
Generally, the width of IR absorption peaks depends on the environment of the target molecule. Intermolecular forces alter vibrational modes. 

If intermolecular attractions are fairly strong (like the hydrogen bonding present in liquid ethanol), the number and strength of interactions can vary greatly from molecule to molecule, causing the vibrational modes observed in an IR absorption spectrum to widen.

As for the reported width of the peak, the convention is to report the width at 1/2 the height of the peak.