Hi Christine,
This seems to be caused by the 'No Printer' option.
When you check the 'No Printer' option, CR doesn't tie the report to a default printer however, when you export the report to Excel the settings are not inherited and Excel seems to pick the settings from the default printer (and hence defaults to A4 Letter).
Uncheck the 'No Printer' option > Check 'Dissociate Formatting Page Size and Printer Size' > Export.
-Abhilash