Webpression is not ideal for huge sites with thousands of pages. While it may work, it may not be as flexible as you require in this type of scenario.
Webpression is primarily a content publishing tool rather than a graphical design tool, so if you are trying to achieve pixel perfect placement of elements on a page, you may be frustrated. The built in WYSIWYG editor is powerful, but it is not equal to a word processor where you can do almost anything you like with formatting and placement of content. While this might seem restrictive, the results will render consistently accross different web browsers and operating systems.
You CAN produce advanced design by making use of the Webpression template system, and the built in CSS styles editor, however the editor is intended for non-technical users who need to be able to change textual content on a day to day basis.