--abr n   Average bitrate encoding

Turns on encoding with a targeted average bitrate of n kbits, allowing to use frames of different sizes. The allowed range of n is 8 to 310; you can use any integer value within that range.

This switch may be combined with the -b and -B switches:

--abr 123 -b 64 -B 192

This would limit the allowed frame sizes to between 64 and 192 kbits.