The size of an MP3 file depends on the bit rate such as 128 kb/sec for
CBR or constant bit rate encoding. Note that this is very different
from the sample rate like 44.1 kHz or 48 kHz which is function of how
the analog signal is digitized. You can raise or lower the bit rate to
trade off quality and file size. Note that if you don't change the bit
rate, the mp3 file size will be the same for mono or stereo. However,
you should be able to reduce the bit rate somewhat for mono and have
the equivalent quality of a stereo file at the higher bit rate. This
will not necessarily be a 2:1 relationship, however, because of the
existing correlation between the two channels improves the sound
quality relative to file size.