It seems to be an issue with RadMessageBox. I have logged it in our feedback portal by making this thread public. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item.
I have also updated your Telerik points.
Currently, the possible solution that I can suggest is to store the initial size of the message and restore it when showing the next times:
public RadForm1(){ InitializeComponent(); RadMessageBox.Instance.SizeChanged += Instance_SizeChanged; RadMessageBox.Instance.FormClosed += Instance_FormClosed;}Size s = Size.Empty;private void Instance_FormClosed(object sender, FormClosedEventArgs e){ s = RadMessageBox.Instance.Size; RadMessageBox.Instance.FormClosed -= Instance_FormClosed;}private void Instance_SizeChanged(object sender, EventArgs e){ if (s != Size.Empty && RadMessageBox.Instance.Size.Width != s.Width) { RadMessageBox.Instance.SizeChanged -= Instance_SizeChanged; RadMessageBox.Instance.Size = s; RadMessageBox.Instance.SizeChanged += Instance_SizeChanged; } Console.WriteLine(RadMessageBox.Instance.Size);}private void radButton1_Click(object sender, EventArgs e){ RadMessageBox.Show(@"<html><size=12>This is RadLabel <br><b><font=Arial>Arial, Bold</b><br><i><color= Red>" + @"<font=Times New Roman>Times, Italic <u>Underline</u><br><size=9>Size = 9<br><color= 0, 0, 255>Sample Text"); }
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I hope this information helps.
Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik