GridCurrentCell cc = e.TableControl.CurrentCell;
if (cc.Renderer is GridComboBoxCellRenderer)
{
GridComboBoxCellRenderer rend = cc.Renderer as GridComboBoxCellRenderer;
GridComboBoxListBoxPart lb = rend.ListBoxPart as GridComboBoxListBoxPart;
lb.Size = new Size(c, 100); //c is width of the column
}