Replace the white box with a white filled table cell (with or without a border).
Table cells (and frames) will adjust to the amount of text. Text boxes won't.
As for ease of access - create a macro to perform the steps e.g. as follows. Change the path and template name as appropriate and add the macro to a button on the QAT (Quick Access Toolbar). The process is virtually identical to that shown at
http://www.gmayor.com/installing_macro.htm (albeit that link shows the process in Word).
Code:
Sub PersonalMessage()
Dim oItem As MailItem
Const strFolder As String = "D:\Word 2010 Templates\"
Const strFilename As String = "Example.oft"
Set oItem = Application.CreateItemFromTemplate(strFolder & strFilename)
oItem.BodyFormat = olFormatHTML
oItem.Display
Set oItem = Nothing
End Sub