///////Begin Source
uses
  Math;

function DBGridRecordSize(mColumn: TColumn): Boolean;
{ 返回记录数据网格列显示最大宽度是否成功 }
begin
  Result := False;
  if not Assigned(mColumn.Field) then Exit;
  mColumn.Field.Tag := Max(mColumn.Field.Tag,
    TDBGrid(mColumn.Grid).Canvas.TextWidth(mColumn.Field.DisplayText));
  Result := True;
end; { DBGridRecordSize }

function DBGridAutoSize(mDBGrid: TDBGrid; mOffset: Integer = 5): Boolean;
{ 返回数据网格自动适应宽度是否成功 }
var
  I: Integer;
begin
  Result := False;
  if not Assigned(mDBGrid) then Exit;
  if not Assigned(mDBGrid.DataSource) then Exit;
  if not Assigned(mDBGrid.DataSource.DataSet) then Exit;
  if not mDBGrid.DataSource.DataSet.Active then Exit;
  for I := 0 to mDBGrid.Columns.Count - 1 do begin
    if not mDBGrid.Columns[I].Visible then Continue;
    if Assigned(mDBGrid.Columns[I].Field) then
      mDBGrid.Columns[I].Width := Max(mDBGrid.Columns[I].Field.Tag,
        mDBGrid.Canvas.TextWidth(mDBGrid.Columns[I].Title.Caption)) + mOffset
    else mDBGrid.Columns[I].Width :=
      mDBGrid.Canvas.TextWidth(mDBGrid.Columns[I].Title.Caption) + mOffset;
    mDBGrid.Refresh;
  end;
  Result := True;
end; { DBGridAutoSize }
///////End Source

///////Begin Demo
procedure TForm1.DBGrid1DrawColumnCell(Sender: TObject; const Rect: TRect;
  DataCol: Integer; Column: TColumn; State: TGridDrawState);
begin
  DBGridRecordSize(Column);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  DBGridAutoSize(DBGrid1);
end;

（出处：http://www.delphibbs.com/delphibbs/dispq.asp?lid=3216893）