Set a “Width Override” on the SizeBox parent of the ListView. This way, the ListView will have better knowledge of its dimensions, and the child widgets will know how much they can expand.
Another thing you could do is set “Wrap Text At” (Text Widget), specifying the width at which the text should wrap. This is approximately the width of the Text widget in the list.
Finally, remember that UMG doesn’t know how large anything is until it renders. You can force Unreal Engine to perform a layout prepass by using the “Force Layout Prepass” node on the widget after adding the children to the ListView.
Source: Nice text wrapping in UMG — Double Cat
I hope this helps.