Leedsichthys is the largest known member of the Osteichthyes or bony fishes, with the exception of the largest descendants of land-dwelling Tetrapodomorpha (e.g. giant sauropods and whales), not commonly thought of as "fishes".
The largest extant non-tetrapodomorph bony fish is the ocean sunfish, Mola mola, being with a weight of up to two tonnes an order of magnitude smaller than Leedsichthys.
The extant King of Herrings might rival Leedsichthys in length but is a much more elongated animal.
The lack of a preserved vertebral column has made it difficult to estimate the exact length of Leedsichthys.
Arthur Smith Woodward, who described the type specimen in 1889, estimated specimen BMNH P.10000 to be of an around nine metre long individual, by comparing this tail of Leedsichthys, having a preserved height of 274 centimetres (108 inches), with another pachycormid, Hypsocormus.
The length of Leedsichthys was not historically the subject of much attention, the only reference to it being made by Woodward himself when he in 1937 indicated it again as 9 metres (29.5 feet) on the museum label of BMNH P.10000.
However, in 1986, David Martill compared the bones of Leedsichthys to a pachycormid that he had recently discovered, Asthenocormus.
The unusual proportions of that specimen gave a wide range of possible sizes.
Some were as low as 13.5 metres (44.3 feet), but extrapolating from the gill basket resulted in an estimated length of 27.6 metres (90.6 feet) for Leedsichthys specimen NHM P.10156 (the earlier BMNH P.10156).
Martill considered the higher estimate as a plausible size of the largest individuals.
Subsequently, a length of thirty metres (hundred feet) was often mentioned in popular science publications, sometimes one as high as thirty-five metres (115 feet).

Liston in his studies concluded to much lower estimates.
Documentation of historical finds and the excavation of "Ariston", the most complete specimen ever from the Star Pit near Whittlesey, Peterborough, support Woodward's figures of between 9 and 10 metres (29.5 and 33 feet).
With "Ariston" the pectoral fins are 1005 millimetres (39.6 inches) apart, indicating a narrow body of no excessive size, even though it was initially thought to have been 22 metres (72 feet) long.
In 2007 Liston stated that most specimens indicated lengths between 7 and 12 metres (23 and 39 feet).
A linear extrapolation from the gill basket would be flawed because the gills grow disproportionally in size, having to increase their surface allometrically to ensure the oxygen supply of a body increasing in volume to the third power.
The growth ring structures within the remains of Leedsichthys have indicated that it would have taken 21 to 25 years to reach these lengths, and isolated elements from other specimens showed that a maximum size of just over 16 m (52 ft) is not unreasonable.
In 2013 a new study by Liston found most specimens to be between 17 and 31 years old.
Specimen GLAHM V3363, "Big Meg", could have been between 11.4–14.9 m (37–49 ft).
The exception was NHM P.10156.
Its gill basket, with a preserved width of 114 centimetres (44.9 inches) and height of 1545 millimetres (60.8 inches), indicated a body length of 16.5 m (54 ft) and an age of 45 years.
Leedsichthys is the largest known member of the Osteichthyes or bony fishes, with the exception of the largest descendants of land-dwelling Tetrapodomorpha (e.g. giant sauropods and whales), not commonly thought of as "fishes". The largest extant non-tetrapodomorph bony fish is the ocean sunfish, Mola mola, being with a weight of up to two tonnes an order of magnitude smaller than Leedsichthys. The extant King of Herrings might rival Leedsichthys in length but is a much more elongated animal. The lack of a preserved vertebral column has made it difficult to estimate the exact length of Leedsichthys. Arthur Smith Woodward, who described the type specimen in 1889, estimated specimen BMNH P.10000 to be of an around nine metre long individual, by comparing this tail of Leedsichthys, having a preserved height of 274 centimetres (108 inches), with another pachycormid, Hypsocormus. The length of Leedsichthys was not historically the subject of much attention, the only reference to it being made by Woodward himself when he in 1937 indicated it again as 9 metres (29.5 feet) on the museum label of BMNH P.10000. However, in 1986, David Martill compared the bones of Leedsichthys to a pachycormid that he had recently discovered, Asthenocormus. The unusual proportions of that specimen gave a wide range of possible sizes. Some were as low as 13.5 metres (44.3 feet), but extrapolating from the gill basket resulted in an estimated length of 27.6 metres (90.6 feet) for Leedsichthys specimen NHM P.10156 (the earlier BMNH P.10156). Martill considered the higher estimate as a plausible size of the largest individuals. Subsequently, a length of thirty metres (hundred feet) was often mentioned in popular science publications, sometimes one as high as thirty-five metres (115 feet).
Liston in his studies concluded to much lower estimates. Documentation of historical finds and the excavation of "Ariston", the most complete specimen ever from the Star Pit near Whittlesey, Peterborough, support Woodward's figures of between 9 and 10 metres (29.5 and 33 feet). With "Ariston" the pectoral fins are 1005 millimetres (39.6 inches) apart, indicating a narrow body of no excessive size, even though it was initially thought to have been 22 metres (72 feet) long. In 2007 Liston stated that most specimens indicated lengths between 7 and 12 metres (23 and 39 feet). A linear extrapolation from the gill basket would be flawed because the gills grow disproportionally in size, having to increase their surface allometrically to ensure the oxygen supply of a body increasing in volume to the third power. The growth ring structures within the remains of Leedsichthys have indicated that it would have taken 21 to 25 years to reach these lengths, and isolated elements from other specimens showed that a maximum size of just over 16 m (52 ft) is not unreasonable. In 2013 a new study by Liston found most specimens to be between 17 and 31 years old. Specimen GLAHM V3363, "Big Meg", could have been between 11.4–14.9 m (37–49 ft). The exception was NHM P.10156. Its gill basket, with a preserved width of 114 centimetres (44.9 inches) and height of 1545 millimetres (60.8 inches), indicated a body length of 16.5 m (54 ft) and an age of 45 years.
Leedsichthys