Cannot resize child widget within scrollbox?

I’m trying to use a text widget inside a scrollbox for a UI driven mobile phone. the problem I’m having is that I can’t set the text widget to be the size of the scrollbox, so if there is too much text inside it pushes against the header at the top rather than going down into the scrollbox.

Is there a way to fix this? or if not is there a better way of displaying text in this scenario?