Create a custom theme starting from Aqua theme. Change the Font for RadLabelElement-TextPrimitive to "Microsoft Sans Serif, 10pt". Save the theme and apply it to the entire application. Show several times a RadMessageBox with long text. The first time the RadMessageBox is sized correcly to its content. Each next showing, cuts off the text.

Workaround:  the possible workaround that I can suggest is to dispose the RadMessageBox instance after showing it:

Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
    RadMessageBox.Show(Me, "If you have some veeeeery long sample text, the messagebox " & _
                       "should be resized according to its content.", "Caption", MessageBoxButtons.OK, My.Resources.image)
    RadMessageBox.Instance.Dispose()
End Sub