The same combobox is used for any cell that has celltype "ComboBox". So, in order to set its width, you need to do it dynamically as each combobox is dropped (to allow different cells to have different widths). You can do this in the CurrentCellShowingDropDown event.
Private Sub grid_CurrentCellShowingDropDown(ByVal sender As Object, ByVal e As GridCurrentCellShowingDropDownEventArgs) Handles grid.CurrentCellShowingDropDown
Dim cc As GridCurrentCell = Me.grdEstimate.CurrentCell
//size the dropdown based on cc.RowIndex and cc.ColIndex...
e.Size = New Size(200, e.Size.Height)
End Sub