Marc-André: estimating the risks of giving up on a valid query for a truly random hash, at an overestimated one billion queries per second, in a 2/3 full dictionary:

* for 1000: 4E159 years between mistakes

* for 100: 12.9 years between mistakes

* for 150: 8E9 years between mistakes

* for 200: 5E18 years between mistakes

So while it seems that 100 might be a bit too small, using 150 to 200 is perfectly safe (and that's "perfect" in the sense that a computer will encounter random hardware errors at a higher rate than that).