Can I remove the white margins around the page in the PDF preview so that the text is larger?

Add a blank file named latexmkrc (no extensions) to your project, and put this line in it:

$pdflatex = 'pdflatex %O %S; pdfcrop --margins "20" %B.pdf %B-c.pdf; cp %B-c.pdf %B.pdf';

(Write $pdflatex = 'xelatex ... ' instead if you want to use XeLaTeX.)

This is a great way to remove the border whilst you're working in Overleaf using e.g. the standard article template. Just remember to rename the file to latexmkrc.txt or delete it when you want to download the final PDF; otherwise the downloaded PDF will also have its margins cropped!

(We don't currently have a zoom mechanism for the preview panel, though it's planned—stay tuned!)