The maximum size of a planet is defined in that it is not large enough
to support fusion.
This would put it smaller than a brown dwarf which I think does output
some energy by small scale fusion occurring of deuterium.

I do not think the orbit should effect the definition, e.g. if the
Earth had been knocked into a different orbit by a collision , it
should still be classified as a planet as long as it did remain in
orbit around the sun.

The term planet does not apply just to the solar system so the naming
rules would apply to objects in orbit around other stars.

Personnally I do not consider Pluto a planet, but as it is generally
accepted as a planet this would be the obvious size to use as the
minimum.