Build 4.0.0.22 of xlsgen exposes the maximum number of rows and columns in each workbook, thanks to the MaxRows and MaxColumns properties available in the IXlsWorkbook interface.

This returns, for any XLS file (and variants such as XLA, XLM, ...), 65536 rows and 256 columns. And it returns 1048576 rows and 16384 columns for any XLSX file (and variants such as XLSM, XLSB, ...)