Black holes are very small for their mass but get bigger faster than normal bodies which assuming an even density their size goes up as the cube root of their mass.

The size of a black hole is a linear function of it mass that is if a black hole with a particular mass is of a particular size one of twice the mass is twice the size. Now a black hole with a mass equal to the mass of the sun is about a mile across, so a black hole with a million solar masses is about the size of the sun, one with one hundred million solar masses is about the size of the earth's orbit around the sun.

A useful reference to give you the properties of black holes is

http://xaonon.dyndns.org/hawking/for small black holes

A black hole one centimetre across has a mass bit more than the earth

A black hole about one tenth of a millimetre across has a temperature about equal to the cosmic microwave background and anything smaller will be slowly evaporating

A black hole one nanometer across (the size of an atom) has a mass of 10^15 metric tons

A black hole 1 femtometer across (the size of the nucleus of an atom) has a mass of 600,000,000 metric tons this is around one cubic kilometre of water. It

has an energy output of around 800 Megawatts (a good sized power station and will last around 800 thousand million years before it evaporates.