[im3w1l]

As an aside, sha-1 is smaller than 128 bytes.

From my numerical experiments (I hope I didn't mess up...) using the random oracle model, the probability that a given key is collision-free is 99.6% if the input is one byte shorter than hash, 1/e if input is same size as hash and 6.6e-112 if the input is one byte longer than hash.

And this holds basically irrespective of key size.