I think most current models are sealed subs. The 18.3 & 15.3 have passive radiators, but I believe all other current models (18.0, 18.0C, 15.0) are sealed.
My F18.0 are sealed. But of course, according to Ridikas, sealed subs are just bad, bad, bad.
Oh, yeah, the OP needs subs, so we are giving him options.
What about the F18.0C, which is 20" x 20" x 18.5", the same size as the F15.0.