If your end users are using Excel 2003 or older, you are going to need to split the output into either
- several different workbooks
- several different worksheets in a single workbook
based on some logical split - e.g. product type(maybe your web app can ask user for split).
If they are using Excel 2007 this restriction largely goes away
MSDN[
^], however you need to remember that if all the data is in one workbook (using the several worksheets, or Excel 2007 approach) it will take a long time to open.
Regardless of which approach you take I would suggest that you design a web front end that accepts the request, which then fires an asynchronous process to export the data and send it to them (perhaps zipped via e-mail) once it has completed.