Recognising what to ignore is the most important step. Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see)

In general, ignore files less than 2,000 KB (2 MB) but also some others. Here's a good place to look for hints: MAJOR TOPICS for Upload/Download HELP (click on blue link)

There are also some audio files that have been 'deliberately' corrupted. They show incorrect bit rate for the size such a file should be for the duration of the song. Or vice versa, wrong size according to the supposed bit rate. I guess it takes experience to get a rough idea what size audio files should be according to their duration & bit rate.