Xara doesn't perform super well on documents with a very large number of pages (content dependant, 1000 pages of text is easier to handle than say 500 pages of images). It's a known performance issue on the list to improve but there's no hard limit on the number of pages you can have in a document. Doesn't come up very often though as the majority of documents I see are less than 100 pages.

As far as memory limits go it's all system dependant. If your on a 32bit system it's 3.5Gb, on 64bit I think it's something huge like 192gb. You can go to File > Document information to see roughly the memory usage of the document.

Thanks
Rob