A part 25 aircraft is one too big to be a part 23 aeroplane. This is true whether you're talking about FAR or JAR. The definition of a part 23 aeroplane is.....
JAR 23.1 - Applicability
Date: March 11, 1994
(a) This code prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for -
(1) Aeroplanes in the normal, utility and aerobatic categories that have a seating configuration, excluding the pilot seat(s), of nine or fewer and a maximum certificated take-off weight of 5670 kg (12 500 lb) or less; and
(2) Propeller-driven twin-engined aeroplanes in the commuter category that have a seating configuration, excluding the pilot seat(s), of nineteen or fewer and a maximum certificated take-off weight of 8618 kg (19 000 lb) or less.
(b) Each person who applies for such a certificate or change must show compliance with the applicable requirements of this code.
So basically anything bigger than that.
G