ExcelFile.HeadingColWidth Property
Width that will be used by the added column with row numbers when PrintHeadings is true. The default value (0) means to use an automatic value which is fine if you don't have too many rows to print. A positive value will make the column wider or narrower. A negative value will auto-calculate the width depending in the number of rows and normal font of the spreadsheet.
Syntax
Namespace: FlexCel.Core
public abstract Double HeadingColWidth { get; set; }