Nick, 2.25" x 3.25" is an american format, as far as I know invented by Kodak for 120 film. But nowadays the world is pretty well metricated, US' backwardness notwithstanding. 35 mm is also an american format, invented jointly by Eastman and Edison. Go figure.
2.25" x 3.25" does not go evenly into the native UK LF film/plate size 1 plate. A plate is 6.5" x 8.5", the smallest commonly used fractional plate is quarter plate, 3.25" x 4.25".
2.25" x 3.25" is commonly approximated in metric by 6x9. This is not a good approximation even though it comes very close to preserving the format's aspect ratio. Can't shoot 6x9 on 120 film, the film's not wide enough.
There was a metric 6.5 x 9 sheet film size that's not used much these days.
None of this matters very much except when trying to compare angles of view given focal length across formats. Then, if we get a format's dimensions wrong we get the equivalent focal length wrong.