DropDownWidth Property
Returns or sets the width (in pixels) of the list for the specified command bar combo box control. Read/write Long.
Note An error occurs if you attempt to set this property for a built-in control.
Remarks
If this property is set to -1, the width of the list is based on the length of the longest item in the combo box list. If this property is set to 0, the width of the list is based on the width of the control.
Example
This example adds a combo box control containing two items to the command bar named "Custom". The example also sets the number of line items, the width of the combo box, and an empty default for the combo box.
Set myBar = CommandBars("Custom")
Set myControl = myBar.Controls.Add(Type:=msoControlComboBox, Id:=1)
With myControl
.AddItem "First Item", 1
.AddItem "Second Item", 2
.DropDownLines = 3
.DropDownWidth = 75
.ListHeaderCount = 0
End With