You can conrol the width of things in CurrentCellShowingDropDown.
private void gridControl1_CurrentCellShowingDropDown(object sender, GridCurrentCellShowingDropDownEventArgs e)
{
GridCurrentCell cc = this.gridControl1.CurrentCell;
if(cc.Renderer is GridDropDownGridListControlCellRenderer)
{
GridDropDownGridListControlCellRenderer cr = cc.Renderer
as GridDropDownGridListControlCellRenderer;
cr.ListControlPart.Grid.ColWidths[1] = 300; //set width of column
e.Size = new Size(400, e.Size.Height); //set width of dropdown
}
}