在combobox所在的窗口的Formshow事件裏調用即可

1. procedure SetComboBoxListWidth( AComboBox: TComboBox );
2. var
3. i: Integer;
4. nMaxLen, nMinWidth: integer;
5. nFontWidth: Integer;
6. nCboLeft: integer;
7. ctlCustom: TControl;
8. begin
9. nCboLeft := AComboBox.Left;
10. ctlCustom := AComboBox;
11. with AComboBox do
12. begin
13. nFontWidth := Round( Abs(Font.Height / 2 ) );
14. nMaxLen:= 0;
15. for i:=0 to Items.Count-1 do
16. begin
17. if length(Items[i])* nFontWidth > nMaxLen then
18. nMaxlen:= length(Items[i])* nFontWidth+5;
19. end;
20. if Items.Count > DropDownCount then
21. nMaxLen := nMaxLen + 20;
22. if nMaxLen > Width then
23. begin
24. if Items.Count>DropDownCount then
25. begin
26. SendMessage( Handle, CB_SETHORIZONTALEXTENT, nMaxLen+5, 0 );
28. {解決分辨率小導致的ComboboxList的寬度超出屏幕}
29. //while 部分是用來取出combobox控件相對與窗體的橫坐標
30. while not (ctlCustom.Parent is TForm) do
31. begin
32. nCboLeft := nCboLeft + ctlCustom.Parent.Left;
33. ctlCustom := ctlCustom.Parent;
34. end;
35. nMinWidth := Min(400, nMaxLen); // 使用不大於nMaxLen的數做比較
36. if (nCboLeft + nMinWidth) > Screen.Width-25 then // 不超出屏幕，並保留窗口滾動條寬度，約25
37. nMinWidth := Screen.Width-25-nCboLeft;
38. nMinWidth := Max(nMinWidth, Width); // 不小於控件自身寬度
40. PostMessage(Handle, CB_SETDROPPEDWIDTH, nMinWidth, 0);
41. end
42. else
43. PostMessage(Handle, CB_SETDROPPEDWIDTH, nMaxLen , 0);
45. ShowHint := True;
46. end
47. else
48. begin
49. SendMessage( Handle, CB_SETHORIZONTALEXTENT, 0, 0 );
50. PostMessage(Handle, CB_SETDROPPEDWIDTH, Width , 0);
51. end;
52. end;
53. end;

http://blog.csdn.net/youthon/article/details/8179348

delphi自動調整combobox下拉列表寬度（PostMessage CB_SETDROPPEDWIDTH）