Check the data type of the field on the Data tab and make sure it's a memo field (the underlying view could be retrieving the truncated string).
Check that the field property on the Design tab has stretch enabled (this is allow the field to grow as necessary).
If the field is within a Region, then check the stretch property of the region as well.
Also note that Excel as a limit on the length of a cell value. So even if the report displays the full text, once exported to excel it may be truncated again.