by grey-hame » 08 Jan 2010 11:20
Images (jpeg, gif, etc.) and music (mp3, ogg) are safe and tend to be smaller than 5MB as well. (Other formats, e.g. tiff, bmp, flac, wav, tend to be bigger, often because they're uncompressed.)
Video smaller than 5MB is often corrupt or spam, but occasionally brief, legitimate clips.
Software smaller than 5MB is usually malware.
Download the RAR and if it's got an executable in it and you weren't looking for software, just delete it without extracting anything from it. It can't actually jump out and infect you; as long as you don't extract and run it you should be safe.