As far as I know every page in Scribus can have its own size?
While this is true, there is a caveat: "Normal" master page is applied to pages by default, so if you change a page size manually via "Page > Manage Page Properties...", and at some point even just open master page settings (Edit > Master Pages...), those manual changes will be reset, and "Normal" master page will be automatically reapplied to all pages. I am not sure if this is a bug or a feature.
Thus, @OP, even if your way of doing it works for you, it is more of a workaround under the circumstances. For the sake of those that might face a similar issue and stumble upon this thread, I'd suggest trying the following:
- create a new master page (Edit > Master Pages... > Add a new master page), which will be used for the first page in the document
- name it something like "Cover Page", and make sure it is selected in "Manage Masterpages" palette
- under "Page" menu choose "Manage Page Properties...", set size and margins, press "OK", and, if nothing else should be added to the page, press "Return to Normal Page Mode"
- go to the first page, click on it to make sure it is active page, open "Manage Page Properties..." in "Page" menu, then choose "Cover Page" under "Master Page" at the bottom of the dialog. Click "OK"
Just as one should always use paragraph and character styles for text, same goes for pages: it is a good idea to use "page styles", which are basically master pages. This is true not only when the size of a page should be different, but in general, whenever a different layout or background is needed (a typical example being "Parts" in a book).