If you see an error message that reads, 'The application has experienced a system error. The reported error message was: Exception of type ‘System.OutOfMemoryException’ was thrown' while importing via a File, this is likely related to the size of your file.
The size of the file may be due to extra rows or columns but it may also be due to something else embedded in the file eg a graphic or other object that is not visible. This may be resolved by saving the Excel file as a CSV and using that as your import file.
To see if this is due to extra rows/columns, open it up in, a notepad to see if there are any extra lines and columns. Normally, even if it is only 100 rows, if there are extra blank rows or columns that can increase the size of the file enormously. If you see blank rows or columns, you should remove those to see if that resolves the issue.