You need to know what is & what isn't junk in order to use it. When the search results come in, select a result that is junk & press the trash button. The spam filter is a learning filter; ie: you need to train it to work how you want it to work. The more similar results you select as junk then after a while the filter will recognise these automatically. You have junk filter options under LW's menu bar, Tools>Options>Filters>Junk
I found that for the autogenerated spam like the ipod & mp3 spam that it was very quick to learn. However for virii files, it takes much longer.

Here's some references for file sizes: 1. (a) autogenerated spam results, & also (b) autogenerated mp3 spam results

2. WARNING: Viruses on network you should be aware of! (click on link)

BTW it works by not only recognising the file size, but also anything else that puts similar files into the same category such as naming technique, etc.