Probably one with under about 980 Newtons weight

Using Hocus Pocus as a data sample, Bette Midler is the average sized witch.

Assuming that’s it’s just the normal force of Earth’s gravity at work on the witch, it’s just a case of working Newton’s second backwards with the gravitational “constant” acceleration ≈ 9.8 m/s²:

F = ma

980 N = m × 9.8 m/s²

980 / 9.8 = (m × 9.8) / 9.8, units omitted for text clarity

100 kg = m

In other words, op is assuming that an average size witch masses approximately 100kg (or about 220 lbs in fingers, knees, and toes units).

About the weight of a duck.