The following error message is displayed when trying to edit an excel spreadsheet:
"Couldn't Open the Workbook. Sorry we can't open your workbook in Excel Online because it exceeds the 25 MB file size limit".
This is a known limitation of online editing, when working with Excel documents their size must be less than 25MB for the application to open them.
Solution: Use local editing with the 'Edit in Excel' option. For local editing to work the client machine must have Excel installed.