Steps at design time:
1.Add a RadGridView to the from and change its Dock property to Fill.
2.Change its AutoSizeColumnsMode property to Fill.
3.Chage the Form.Size property to Width = 527 and Height = 346.

Use the following code:
public Form1()
{
    InitializeComponent();

    DataTable dt = new DataTable("Items");

    dt.Columns.Add("Id", typeof(int));
    dt.Columns.Add("Description", typeof(string));
    dt.Columns.Add("Price", typeof(decimal));
    dt.Columns.Add("Supplier", typeof(string));

    for (int i = 0; i < 10; i++)
    {
        dt.Rows.Add(i, "Description" + i, i * 0.25, "Supplier" + i);
    }

    radGridView1.DataSource = dt;
}

Workaround: set the AutoSizeColumnsMode property to Fill in the Form.Load event:
private void Form1_Load(object sender, EventArgs e)
{
    radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
}