VB.NET - Autosize font to fit multiline textbox

Got it! Kschuler, you pointed me in the right direction!
Here is a quick sloppy sample I created, when the Button is clicked, the Rich Text Box is adjusted to fill the entire box.

Public Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" (ByVal hwnd As IntPtr, ByVal nIndex As Integer) As Integer
    Public Declare Function GetSystemMetrics Lib "user32.dll" (ByVal nIndex As Integer) As Integer
    Public Const GWL_STYLE As Integer = (-16)
    Public Const WS_VSCROLL As Integer = &H200000
    Public Const WS_HSCROLL As Integer = &H100000
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'Center the text in the RichBox
        RichTextBox1.SelectAll()
        RichTextBox1.SelectionAlignment = HorizontalAlignment.Center
        'Is scrollbar visible?
        Dim bVScrollBar As Boolean
        bVScrollBar = ((GetWindowLong(Me.RichTextBox1.Handle, GWL_STYLE) And WS_VSCROLL) = WS_VSCROLL)
        Select Case bVScrollBar
            Case True
                'Scrollbar is visible - Make it smaller
                Do
                    RichTextBox1.ZoomFactor = RichTextBox1.ZoomFactor - 0.01
                    bVScrollBar = ((GetWindowLong(Me.RichTextBox1.Handle, GWL_STYLE) And WS_VSCROLL) = WS_VSCROLL)
                    'If the scrollbar is no longer visible we are done!
                    If bVScrollBar = False Then Exit Do
                Loop
            Case False
                'Scrollbar is not visible - Make it bigger
                Do
                    RichTextBox1.ZoomFactor = RichTextBox1.ZoomFactor + 0.01
                    bVScrollBar = ((GetWindowLong(Me.RichTextBox1.Handle, GWL_STYLE) And WS_VSCROLL) = WS_VSCROLL)
                    If bVScrollBar = True Then
                        Do
                            'Found the scrollbar, make smaller until bar is not visible
                            RichTextBox1.ZoomFactor = RichTextBox1.ZoomFactor - 0.01
                            bVScrollBar = ((GetWindowLong(Me.RichTextBox1.Handle, GWL_STYLE) And WS_VSCROLL) = WS_VSCROLL)
                            'If the scrollbar is no longer visible we are done!
                            If bVScrollBar = False Then Exit Do
                        Loop
                        Exit Do
                    End If
                Loop
        End Select
    End Sub

There is no function to tell you what the font size should be to get everything to fit.

I had to do something similar, but without the requirements of editing the text in the box. What I did was make a custom control that displayed the text painted in a panel. But, in order to get the text to fit properly without constantly guessing at the right font, I painted the text oversized in an offscreen buffer scaled up from teh actual size of my rendering area. Then I just painted the picture scaled down to fit the width or height of the render area, depending on which was greater.