Jamieson, Fausset and Brown on Zechariah 5:2

2. length ... twenty cubits ... breadth ... ten cubits--thirty feet by fifteen, the dimensions of the temple porch (1Ki 6:3), where the law was usually read, showing that it was divinely authoritative in the theocracy. Its large size implies the great number of the curses contained. The Hebrew for "roll" or "volume" is used of the law (Psa 40:7).