we have a slightly smaller project (about 30 pages) built on Caliburn. As I see, the only complication for more pages would be memory consumption, since the caliber in its finished behavior initializes all pages (screens / viewing modes) and saves them in memory. We created our own way of processing this type of "lazy screen guides", which creates a viewmodel only when its page is requested, and there is a way to close it (and thus utilize the garbage collector). So now it doesn’t matter if there were 30 or 300 pages in the application. It will consume as much memory as necessary for open pages (provided that the user does not need all 300 pages at once).
Btw: Caliburn.Micro, . , Caliburn.Micro ( , Caiburn), , .