First off, Nymphs are classically always female. In BU, however, this does not have to be the case. They are also not related to the term nymphomaniac. If you would like to be a celibate, abstinent, or otherwise, feel free to be. You do not have to be a major flirty to be a nymph.
That being said, Nymphs are tied directly to nature. Like dryads and elementals, they are never found far from what binds them. Unlike these other two races, Nymphs are typically tied to a particular geographic location, be it a lake, a hill side, a forest, a mountain and so forth. There, they tend to be responsible for the care of the plants and animals that inhabit the Nymph's domain.
They are, nearly without an exception, incredibly beautiful creatures and often become muses for artists of all types.
All things in nature have a beauty to them, though not all things in nature are beatiful to our eyes. A nymph is one of nature's many compromise to this situation. Most Nymphs were created by the old, primal god of nature as a means to keep up the health of certain areas while the god worked on others or rested. Eventually there came a point where the old god no longer needed their assistance, but rather than abolishing their purpose, they were allowed to continue their guardianship roles.
Nymphs can literally exist anywhere, so long as they are designed for that certain terain. A water nymph that inhabits a lake will not do well and can pluasible die in an hot and/or arid climate such as a desert or near a volcano. Likewise for a forest nymph. (There are no hybrid nymphs either i.e. no desert/forest nymphs and so forth)
While they are not immortal, Nymphs have a life span that stretches centuries, allowing them to live long lives.
Nymphs never become taller than humans. So think realistically in this regards.
Whatever a healthy weight for your height would be.
Nymphs have no need to eat food. They literally live off of the land that they were designed to inhabit and protect.
Nymphs cannot beget more Nymphs, even if they mate with another Nymph. In cases of interspecies mating, the child will be a normal, ordinary race of the non-nymph parent (no half-breeds here). In the case of a Nymph mating with another Nymph, a basic humanoid race (human, elf, halfling or so on) will be born. The only way to become a Nymph through lineage, is to have a Nymph pass on their powers to their child, who would then take responsiblity and guardianship of the location the dying Nymph protected.