Geographically Micronesia lies west of Polynesia, north of Melanesia and east of the Philippines. That puts it primarily in the North Pacific Ocean, though its southernmost islands straddle the equator. It incorporates thousands of small islands and islets, with a land area totaling about 1200 sq. mi., covering over 3 million square miles of water in the western Pacific Ocean.

In order of size, the island groups/countries that make up Micronesia are:

Kiribati 313 sq. mi.; population 102,000

An independent nation straddling both the equator and the International Date Line, Kiribati is the only country in the world that falls in all four hemispheres. 33 coral atolls, 21 of which are inhabited, spread over 1.3 million square miles. One of the poorest countries in the world it has minimal tourist facilities. Popular tourist activities include sailing, snorkeling, and exploring the local culture.

Federated States of Micronesia 270 sq. mi.; population 111,000

An independent country in the Caroline Islands made up of four primary island groups (Yap, Chuuk, Pohnpei, and Kosrae) with over 600 islands (only 65 of which are inhabited), spread over 1,000,000 sq. mi. of ocean east of Palau and the Philippines. Due to its remoteness, there are minimal tourist facilities on these islands. Popular tourist activities include scuba diving, especially in Truk Lagoon with a sunken Japanese fleet, snorkeling, and exploring the local culture.

Guam 210 sq. mi.; population 186,000

A territory of the United States (and one of its primary military bases in the Pacific), Guam is the largest single island in Micronesia, and the southernmost island in the Marianas Archipelago, about three quarters of the way from Hawaii to the Philippines. Guam has a flourishing tourism business comprised mostly of Japanese and other Asian visitors. Popular tourist activities include scuba diving, snorkeling and other water sports, duty free shopping, hiking and visiting historical sites.

Northern Mariana Islands 179 sq. mi.; population 45,000

A commonwealth, or territory, of the United States, the Northern Mariana Islands consist of 14 islands, only three of which are inhabited; southern islands are limestone with fringing coral reefs while the northern islands are volcanic. Popular tourist activities include scuba diving, snorkeling and other water sports, golfing, gambling and visiting historical sites.

See more here:

List of the Islands of Micronesia