Audition opens all files as .wav files. These are uncompressed from the original .mp3. The maximum size of a .wav file is 2GB (just over 3 hours of CD quality audio). So there is no way that Audition can open your file even if you can make it understand the file format properly. You have to truncate it before presenting it to Audition. Something like MP3Cutter should be able to do that for you.
Thanks Ryclark! Just understanding that Audition opens files as .wav goes a long way to understanding the problem. I'll check out MP3Cutter, it sounds like it will do the trick.