Is writeLaTeX limited to 30-pages? The preview stops at that point.
No, you can use writeLaTeX for documents of any length, and the export to pdf will always contain the full length document. The 30-page limit on the preview pane is only for anonymous users; it is raised to 90 pages if you are signed up and signed in.
If you have a specific need for a longer preview please get in touch using our contact page.