Private Declare Function MoveWindow Lib "user32" (ByVal hwnd As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal bRepaint As Long) As Long
Private Sub Form_Load()
Dim i As Integer ' Заполняем комбобокс значениями
For i = 1 To 50
Combo1.AddItem Format$(i)
Next i
Combo1.ListIndex = 0 ' Меняем размер выпадающей части комбобокса
ReSizeCombo Me, Combo1
End Sub
Public Sub ReSizeCombo(frm As Form, cbo As ComboBox) ' Процедура меняет размер выпадающей части комбобокса
Dim cboLeft As Long
Dim cboTop As Long
Dim cboWidth As Long
Dim cboHeight As Long
Dim oldScaleMode As Long ' Меняем Scale Mode формы на пиксели.
oldScaleMode = frm.ScaleMode
frm.ScaleMode = vbPixels ' Сохраняем размеры комбобокса
cboLeft = cbo.Left
cboTop = cbo.Top
cboWidth = cbo.Width ' Задаем новую высоту комбобокса, в соотвествии с рабочей областью формы
cboHeight = frm.ScaleHeight - cbo.Top - 5
frm.ScaleMode = oldScaleMode 'и собственно ресайзаем
MoveWindow cbo.hwnd, cboLeft, cboTop, cboWidth, cboHeight, 1
End Sub
Private Sub Form_Resize()
ReSizeCombo Me, Combo1
End Sub

Следующий пример покажет, как можно задать количество открываемых строк элемента ComboBox

Private Declare Function MoveWindow Lib "user32" (ByVal hwnd As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal bRepaint As Long) As Long
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
Const CB_GETITEMHEIGHT = &H154
Private Sub Form_Load()
Dim i As Integer
'Заполняем комбобокс значениями
For i = 1 To 50
Combo1.AddItem Format$(i)
Next i
Combo1.ListIndex = 0
'Меняем размер выпадающей части комбобокса
ReSizeCombo Me, Combo1, 9
End Sub