You can't
Just sort on 2 mutually exclusive traits.
# of pages has nothing to do with Series
You can do 2 steps:
Select books (for any reason
):
Then sort on the page count.
The sort method Calibre uses ALLOWS bias when the (last done) sort has the same EXACT value.
eg
4 books have the EXACT same page count.
If I sort by Author, THEN page count.
The Authors will APPEAR in the previous order FOR ONLY BOOKS WITH THE SAME # OF PAGES. If there are other books by the same author, those will land at the Page count location (but will also have the bias)