Of the 'normal' type stars, ones that generate their energy through nuclear reactions in their cores, the smallest stars are about a tenth the diameter of the Sun. These stars are usually very faint. On the other end of the scale are the supergiant stars which can be almost 800 times larger than the Sun. Stars this large are usually very bright. There are other types of stars that are left over when a star runs out of nuclear fuel.
Black holes can be almost any size, from a few miles to as large as the solar system.
This question was submitted by Tony at Windham Primary School, Maine. 890 degrees Fahrenheit. The Greenhouse Effect. Venus has a very thick atmosphere of carbon dioxide which very effectively traps heat from the Sun and makes the temperature rise. Ask a High Energy Astronomer .