Enabler can handle anything you have in your spreadsheet. So the limit is actually in the amount of rows you can have in an Excel spreadsheet. For Excel 2010 the number is 1,048,576.
You will notice things become slower with larger numbers of records because it takes time to read a huge spreadsheet, upload it to Salesforce.com, and then update the results.
You will also use a lot of your API limit if you choose to deal with a lot of rows of data. Remember, all Salesforce.com operations (Delete/Insert/Update/Upsert) are performed in batches, and the maximum batch size is 200 records (adjustable in the Settings dialog box). So if you have 100K rows, Enabler would need to perform a minimum of 500 API calls to Delete/Insert/Update/Upsert all of them. Consider checking the Bulk API checkbox when uploading large amounts of data. It will still not be instantaneous, but you’ll at least save your API quota (it’s 10000 rows per API request with Bulk API).