On 2009-6-17 I ran a program that looked through all the ABC  "tunes"
downloaded  by the most recent run of abcbot, the search program that
builds the index for my ABC Tune Finder.  There  were  408394  tunes,
whose  sizes  ranged  from  18  to  14216  bytes.  The three standard
averages for the distribution were:

  mode: 274 (the most common size)
median: 374 (the middle size in the sorted list)
  mean: 435 (total bytes divided by number of tunes)

A good summary might be to say that the average ABC tune on  the  web
is 300 to 400 bytes, with a rather wide variation.

On average, there are are about three tunes per  file,  so  the  mean
file size is somewhat over 1000 bytes.  There are a few files of more
than a megabyte, though, since a few people like  to  put  all  their
tunes  in  a single file.  But there are many more people who like to
have every tune in a separate file, to be combined as they like  when
they  make  up  pages  of  tunes for some event.  The median and mode
tunes-per-file are both 1.