Having come across this error a lot, its actually on the source connection, there must be data that is greater than the column length definition.

One way you can check this is to get a copy of the excel sheet you are importing, insert a new column next to the one that is causing you a problem and type in =Len(<cell ref>) and copy this through all columns, then filter the sheet on anything that is bigger than the column definition in SSIS.

It will also give you an Idea what you max column width should be.