When you have no limited length for a D6 field, you end up with varchar(128), not a logtext, so the import fails as the data won't fit, added a field setting in a hook_install to fix this (or could be added before exiting D6 to avoid this?)

function xxx_update_7025() {
  // fix field "field_oage_summary" added value "2048"
  db_merge('content_node_field')->key(
    array(
      'field_name' => (string) 'field_oage_summary',
    )
  )->fields(
    array(
      'type'   => (string) 'text',
      'global_settings'   => (string) 'a:4:{s:15:"text_processing";s:1:"0";s:10:"max_length";s:4:"2048";s:14:"allowed_values";s:0:"";s:18:"allowed_values_php";s:0:"";}',
      'required'     => (int) 0,
      'multiple '   => (int) 0,
      'db_storage'   => (int) 0,
      'module ' => (string) 'text',
      'db_columns' => (string) 'a:1:{s:5:"value";a:5:{s:4:"type";s:4:"text";s:4:"size";s:3:"big";s:8:"not null";b:0;s:8:"sortable";b:1;s:5:"views";b:1;}}',
      'active '   => (int) 1,
      'locked  '   => (int) 0,
    )
  )->execute();
}