The mature papaya fruit is approx. 4 to 8 inches. long and weighs about 10-12 ounces on up to 2 to 3 lbs.depending upon the type. It has a very high vitamin C content  and has an enzyme (papain) that is used extensively as a meat tenderizer, and to some degree in the beauty industry. It is also some parts of the world to treat intestinal parasites

The papaya tree grows to about 15 feet  tall and the trunk has a
diameter of about 10 cm. The tree generally consists of a single trunk with
extensive leaf scars and generates branches only as a result of injury.
Leaves form a spiral cluster around the upper portions of the tree providing
ample shade for the delicate fruit attached to the trunk. The leaves have a
life of up to six months and and extend about 3 feet from the main stem

The papaya fruit we are most familiar with weighs about 10 - 16 ounces.
The skin is yellow when ripe and green when immature. The soft fruit can
be easily cut to reveal a red pulp protecting a tight cluster of small black
oval seeds.

There are two types of papayas, Hawaiian and Mexican. The Hawaiian varieties are the papayas commonly found in supermarkets. These pear-shaped fruit generally weigh about 1 pound and have yellow skin when ripe. The flesh is bright orange or pinkish, depending on variety, with small black seeds clustered in the center. Hawaiian papayas are easier to harvest because the plants seldom grow taller than 8 feet. Mexican papayas are much larger the the Hawaiian types and may weigh up to 10 pounds and be more than 15 inches long. The flesh may be yellow, orange or pink. The flavor is less intense than that the Hawaiian papaya but still is delicious and extremely enjoyable. They are slightly easier to grow than Hawaiian papayas. A properly ripened papaya is juicy, sweetish and somewhat like a cantaloupe in flavor, although musky in some types. The fruit (and leaves) contain papain which helps digestion and is used to tenderize meat. The edible seeds have a spicy flavor somewhat reminiscent of black pepper. 

Here are a few good papaya links:
http://www.exportjamaica.org/papaya/
http://postharvest.ucdavis.edu/Produce/ProduceFacts/Fruit/papaya.html
http://www.rain-tree.com/papaya.htm