And what determines the size of the focal plane?
There is a formula somewhere that tells you what the theoretical size of the focal plane is, but in practice it is the design of the telescope that determines the size of the user accessible focal plane.
Earlier, I mentioned that a typical Newtonian designed for visual use will typically have a 10mm fully illuminated circle. Inside this circle, the user gets access to the full aperture of the telescope and anything inside that circle is seen at 100% brightness.
Outside of that circle, you still see a field, but it is very slightly vignetted, meaning you are not using the full aperture of the telescope, but unless the illumination falls off sharply the difference in brightness is difficult to see.
What defines the maximum size of the field (the most field the user can access) is the field stop size of the eyepiece and the point where the illumination falloff becomes objectionable.
For example, you could use a 56mm Plossl with an f/5 telescope, but the illumination falloff would probably be intrusive, Some people would say that you can't use a 56mm eyepiece in an f/5 telescope becuase the exit pupil would be to big, but that does not stop you from using a 56mm eyepiece to get a wider field of view. The aggressive illumination falloff near the edge of the field though would probably give the field an unpleasant look.
Things that influence how much illumination falloff there is will be things like the speed of the system, the presence of a baffle like in SCTs and MCTs, the diameter and length of the focuser tube in a refractor, and the size and diameter of the focuser tube in a reflector.
So, there is a theoretical number for the maximum field of an instrument, but in practice, you have to use the amount of field that the observer could actually utilize and this is typically less than the formula value.