The easiest way to get a bigger preview pane is to close off any of the working areas you aren't using at the time. For example if you are working on a clip, you can close down the audio working area by clicking on the word Audio (or the triangle near it) at the top of the Audio
working area. Similarly if you click on the word Sequence you can choose to close down the Clip edit area and only work with the Sequence working area open. Bring back the clip working area by clicking anywhere in the Clips area to the left of the preview area.
Bruce