Hypsilophodon was a relatively small dinosaur, though not quite so small as, for example, Compsognathus.
For Hypsilophodon often a maximum length of 2.3 metres is stated.
This has its origin in a study of 1974 by Galton, in which he extrapolated a length of 2.28 metres based on specimen BMNH R 167, a thigh bone.
However, in 2009, Galton concluded that this femur in fact belonged to Valdosaurus and downsized Hypsilophodon to a maximum known length of 1.8 metres, the largest specimen being NHM R5829 with a femur length of 202 millimetres.
Typical specimens are about 1.5 metres long.
Hypsilophodon would have reached up to half a metre in height.
In 2010, Gregory S. Paul estimated a weight of 20 kilograms (44 lb) for an animal two metres in length.

Like most small dinosaurs, Hypsilophodon was bipedal: it ran on two legs.
Its entire body was built for running.
Numerous anatomical features aided this, such as: light-weight, minimized skeleton, low, aerodynamic posture, long legs, and stiff tail — immobilized by ossified tendons for balance.
In light of this, Galton in 1974 concluded it would have been among the ornithischians best adapted to running.
Despite living in the last of the periods in which non-avian dinosaurs walked the earth, the Cretaceous, Hypsilophodon had a number of seemingly "primitive" features.
For example, there were five digits on each hand and four on each foot.
With Hypsilophodon the fifth finger had gained a specialised function: being opposable it could serve to grasp food items.
Hypsilophodon was a relatively small dinosaur, though not quite so small as, for example, Compsognathus. For Hypsilophodon often a maximum length of 2.3 metres is stated. This has its origin in a study of 1974 by Galton, in which he extrapolated a length of 2.28 metres based on specimen BMNH R 167, a thigh bone. However, in 2009, Galton concluded that this femur in fact belonged to Valdosaurus and downsized Hypsilophodon to a maximum known length of 1.8 metres, the largest specimen being NHM R5829 with a femur length of 202 millimetres. Typical specimens are about 1.5 metres long. Hypsilophodon would have reached up to half a metre in height. In 2010, Gregory S. Paul estimated a weight of 20 kilograms (44 lb) for an animal two metres in length.
Like most small dinosaurs, Hypsilophodon was bipedal: it ran on two legs. Its entire body was built for running. Numerous anatomical features aided this, such as: light-weight, minimized skeleton, low, aerodynamic posture, long legs, and stiff tail — immobilized by ossified tendons for balance. In light of this, Galton in 1974 concluded it would have been among the ornithischians best adapted to running. Despite living in the last of the periods in which non-avian dinosaurs walked the earth, the Cretaceous, Hypsilophodon had a number of seemingly "primitive" features. For example, there were five digits on each hand and four on each foot. With Hypsilophodon the fifth finger had gained a specialised function: being opposable it could serve to grasp food items.
Hypsilophodon