Historically, Excel had a limit of 256 characters per cell (up to Excel 2003 if I remember well).
The following notes are of some interest:
Note 700206 - XXL: MAX Size Limit for Excel Export for Table and Pivot
> "The Maximum limits with XXL Export: Number of Rows - 65536. Number of Columns - 256. Size of individual cell - 255 Characters."
Note 1429136 - OFFI: XXL_FULL_API Support for 1048576 rows in Office 2007
> Since, in Office 2007 Excel supports 10,48,576 rows, with this note we extend limit for XXL Export.
(Note that XXL_FULL_API is based on DOI, what you use).
So I think you can't get more from SAP for now.
Maybe OLE2 (the underlying technology used by DOI) has not this limit of 256 characters, but I wouldn't advise to use it because you might risk performance issues (unless you know how to use the Data Provider to transport big amount of data from frontend to SAP, if I understood well how it works).