the negs on mine measure 2 1/4" x 6 7/8"
2 - no you won't expose film - at least I haven't
Don't know about #'s I turn the knob a certain amount - see my web page on baby al vistas and experiment
http://www.jamieyoungphoto.com/alvistacameras1.html
I figured my f stop at f 14
I haven't had problems with the rails but the film hangs up sometimes at the end on mine - I have to fix it.
Jamie