we're actually talking about the base connection in connection manager, as far as im aware they dont use nvarchar(MAX), as a data type its likley to be DT_WSTR.

if you open that and look at the column thats giving you a problem you will probably see the size is set to a DT_WSTR with a length of 50, increase this to be the correct field length.

After this you will then see a warning triangle on the Excel connection task, simply double click this to open it and refresh the metadata within the data flow, it will probably warn you that the data types are going to be changed, then close it down.

You can check this has changed by viewing the meta data of the Data flow and making sure that the column has actually changed to match the value in the source.