Does the Seneca take modern 5x7 holders or plate holders? If it takes modern film holders you're fine.
5x7" paper is "full size", i.e. just as large as the original 5x7" plates. And yes, 5x7" paper will fit in a 13x18cm holder.
-- Ole Tjugen, Luddite Elitist
I agree with Ole's original observation from experience. 5x7 film ends up inside your bellows (and outside the 13x18 holder) once you re-insert the darkslide!