Вы должны установить максимальную высоту ScrollViewer и могли бы установить Видимость для полос прокрутки на Авто.
Вот пример из msdn :
<ScrollViewer Height="200" Width="200" HorizontalScrollBarVisibility="Auto" Canvas.Top="60" Canvas.Left="340">
<TextBlock Width="300" TextWrapping="Wrap"
Text="I am the very model of a modern Major-General, I've information vegetable, animal, and mineral, I know the kings of England, and I quote the fights historical, From Marathon to Waterloo, in order categorical; I'm very well acquainted, too, with matters mathematical, I understand equations, both the simple and quadratical, About binomial theorem I'm teeming with a lot o' news, With many cheerful facts about the square of the hypotenuse." />
</ScrollViewer>