I'm not sure it's possible to set a column to autofit, from the docs:

"The width corresponds to the column width value that is specified in Excel. It is approximately equal to the length of a string in the default font of Arial 10. Unfortunately, there is no way to specify "AutoFit" for a column in the Excel file format. This feature is only available at runtime from within Excel."

Update: Given that you can determine the maximum string length perhaps you could try adding some padding depending on the font/size being used.