When importing from an Excel file, Toad is truncating data when it shows the Preview screen. On closer inspection, it appears that the wrong columns are being mapped i.e. B is mapping to C, C is mapping to D. This results in data truncation when the wrong table column is specified.
First column in Excel file is empty (null)
Open the Excel file and on the first cell, <right click> Format cells
Selected 'Custom' which highlights 'general'
Repeat Import Table Data Wizard
Waiting for fix in a future release of Toad for Oracle