You can change H and p in the text boxes above and recalculate. The size of a minor planet was estimated using the following equation:
H is the absolute magnitude of the minor planet. This is the magnitude that it would have if it was 1 AU from the Earth and 1 AU from the Sun while having a phase angle of 0 degrees. This is an impossible scenario but it is just the definition of H. This is different than the absolute magnitude used for stars. More Information
p is the geometric albedo of the minor planet. Perfect reflectors of light have A=1 and perfect absorbers have A=0. Typical minor planets have a range of values: 0.05 > A > 0.25 More Information
D is the diameter of the minor planet in kilometers.
The estimates are also available in a table from the MPC.
See also:
http://neo.jpl.nasa.gov/risk/doc/palermo.pdf