Some more US long single-unit diesels (for their time, at least) ...
Baldwin Centipede
Length 91 ft 6 in (27.89 m)
Dual engine, 3000 hp, 1945-1948.
Fairbanks-Morse Trainmaster
Length 66 ft 0 in (20.12 m)
Single engine, 2400 hp, 1953-1957,
EMD/Progress Rail
SD-70ACe-T4
Frame length 76 ft. 8 in / 23.37 m for current (Tier 4 emissions) model, longer than the original SD70 model (72 feet, 4 inches) overall. Single engine, 4400 hp, 1992-present.
GE U50
Length 83 ft 6.5 in (25.46 m)
Dual engine, 5000 hp, 1963-65.
One presumes the OP has single-unit locomotives in mind, not MU lashups that can be of arbitrary length depending on rules and the reliability of control connections.