To reproduce:

public Form1()
{
    InitializeComponent();

    RadMessageBox.Instance.AutoSize = true;
}

private void radButton1_Click(object sender, EventArgs e)
{
    string msg = string.Format("Test", (sender as RadButton).Name);
    RadMessageBox.Show(msg, "Error");
}

Note that the size of  RadMessageBox is adjusted automatically to display the whole content. Hence, it is not necessary to set the RadMessageBox.Instance.AutoSize property to true.