Though that will delete everything, leaving you to have to set up the app again.
Otherwise, how much is cache? You can delete that in the same way (just select "cache" rather than all data), but that won't affect anything important. Or you can see what Settings Outlook has for limiting how much it keeps on your phone (e.g. whether it has options to just keep say the last week's emails on the phone rather than every email you've ever received, that sort of thing). Unfortunately I don't use this app so don't know what options it has.