Rose,
It depends on the species of Leporinus. The one in question (I believe to be Leporinus fasciatus) can get BIG!!! I had two that were 13" at the time I donated them to a local aquarium. Most books state the obvious size in nature, but given the fact that we feed our fish daily, and the good care they receive, they can grow larger than in nature.
Several other species are:
Leporinus friderici - 12" in nature
Leporinus melanopleura - 10" in nature
The fasciatus is a peaceful fish when small, but given it's potential adult size and tank size requirements as well as it's diet (I fed mine algae that would grow on the inside of the glass tops) it really is a specialized species.
They are also very active fish and need to be kept with other fish of equal temperament.
Given the proper tank size, diet and water conditions Leporinus sp. can make a very attractive display.
NOTE: When I gave mine to the aquarium and they were released after proper acclimation, boy was I suprised. These two fish were in a 135 gallon tank and they looked huge. They went into a tank around 2500 gallons at the aquarium and looked like what two guppies would look like in a 135 gallon tank.