For regional rolling stock (with small noses and intended to carry standees) anything not shorter than a tube train can be considered a long train.
Though these, at 121 m, are shorter than some Tube trains:
2009 Stock, 8-car (Victoria line): 133.275 m
1992 Stock, 8-car (Central line): 133 m
1967 Stock, 8-car (Victoria line): 128.2 m
1996 Stock, 7-car (Jubilee line): 126.492 m
1959 Stock, 8-car (Central line): 126.12 m
1960 Stock, 8-car (Central line): 125.88 m
1962 Stock, 8-car (Central line): 123.68 m
S8 Stock (not sure if you’re counting Sub-Surface trains or not, Metropolitan line): 133.682 m
A60/A62 Stock, 8-car (Metropolitan line): 129.24 m
CO/CP/R Stock, 8-car (peak times, Sub-Surface lines): 124.32 m
Any future order of 2024 Stock for the Central line, which would probably be 11-car, would be longer than 121 m as well, since a 10-car unit would already be a minimum of 123.608 m, and the 11th car would be a longer non-bridge car, and, depending on the type of non-bridge car which they use, an 11-car unit would be 133.876 m (using a second Motor 1 or Key Motor 1 car) or 137.546 m (using a second Key Motor 2 car) - presumably it would be at least as long as the 1992 Stock currently used on the line, if not a bit longer.