You can adjust the position of the vertical divider bar (located in between the writing pane and the preview pane) to alter the size of the displayed PDF. You can also use the two arrow buttons at the top of the vertical slider to maximize the view of the PDF or the source code.
More zoom tools are coming in V2—checkout our blog for the latest news and updates.
PS: If you're looking to remove the white margins / border around the page in the PDF preview so that the text is larger on e.g. the standard article template, there's a quick way to do this using
latexmkrc, as described here.