I also have an 11x14 Seneca. I have used the following lenses on it: (i)the lens I use the most is a 12"/21"/28" Turner Reich Triple convertible. The 12" will cover, although I have heard some people claim they won't. Mine does. (ii) a 480mm APO Ronar; (iii)a 600mm Nikkor, (iv) a 30" Kodak Process Ektar.
i use a wollensak 1a triple (13,20,25 ) and it covers ( 11x14 ) at 13" too ... with lots of room to spare