The problem with what you want is that sorting is based in information for each book. But, you want to sort based on information in some other book. To achieve what you want, you would need to add the page count for the first book in the series to the other books in the series. From memory, there is a plugin that can do something like this. I think it has can add information to all books by an author.
I also have to ask, why do you want this? I can't think of how this would be useful. I can see sorting by the total page count for a series, but, I can't see a reason to sort by the page count of the first book in a series.
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