How to set initial size of a TreeViewer?

Question

Hello,

I'm using a TreeViewer within a jface WizardPage and the initial input into the tree causes the WizardPage to grow vertically so that it can show all of the tree's values. When expanding one of the tree's values, then the vertical scrollbar works as expected. I'd like to be able to set the tree's size initially so that it is fixed and the scrollbar is already shown when the WizardPage is first drawn, but doing this isn't particularly obvious to me - the setSize method on the TreeViewer's Tree doesn't seem to do anything.

Any help would be appreciated!