In the renderer''s constructor, try subscribing to the grid''s CurrentCellShowingDropDown event. (It is this event where you can control the dropdown''s size.
public DropDownGridCellRenderer(GridControlBase grid, GridCellModelBase cellModel)
: base(grid, cellModel)
{
this.DisableTextBox = true;
DropDownButton = new GridCellComboBoxButton(this);
this.grid = null;
grid.CurrentCellShowingDropDown += new GridCurrentCellShowingDropDownEventHandler(grid_CurrentCellShowingDropDown);
}
private void grid_CurrentCellShowingDropDown(object sender, GridCurrentCellShowingDropDownEventArgs e)
{
e.Size = new Size(this.Grid.Width, this.Grid.Height);
}