I have noticed that in the crystal viewer some fields would become wider to accomidate the string length (not set to can grow for wrapping but rather extending the right hand side of the field).Perhaps something similar is happening for you.
Here are a few ideas to try.
1. You can try and set all the fields to can grow so they would wrap down. I know that will cause other issues but at least you might know what is not fitting.
2. Use left(field,length) formulas for any field you think might push over the width to make sure it does not.
3. Count the string length of fields you suspect and call those int he report to see if you can force the error.
4. Try and trim all the fields as sometimes users like to throw a lot of spaces or carriage returns into fields.
These are all guesses as I have not personnaly had this error but maybe it will help...