To reproduce:
private void Form1_Load(object sender, EventArgs e)
{
    GridSearchRowElement searchRow = null;
    foreach (GridRowElement row in this.radGridView1.TableElement.VisualRows)
    {
        if (row is GridSearchRowElement)
        {
            searchRow = row as GridSearchRowElement;
            break;
        }
    }
    if (searchRow != null)
    {

        searchRow.SearchCellElement.SearchBoxWidth = 400;
    }
}

Workaround:
radGridView1.TableElement.InvalidateMeasure(true);           
radGridView1.TableElement.UpdateLayout();