There are 2 types of monkeys. One type is New World monkeys and even though they evolved originally in Africa, they are now only found in Central and South America. They somehow migrated from Africa to South America even though the two continents are separated by the Atlantic Ocean. We still do not know how they did it. Anyway, after they migrated, their relatives in Africa were then outcompeted by a newer group, the Old World monkeys. They Old World monkeys are not found in the Americas.
Apes and humans are more closely related to Old World monkeys than to New World monkeys. Most Old World monkeys are medium to large, and the Talapoin is the smallest, being about 12-17 inches in length. Some New World monkeys, such as tamarins, can be small but some, like the Howler monkeys, are much larger. Monkeys are higher primates. The tarsier is not a monkey, but it is a higher primate. It is also pretty small. only about 4-6 inches long.
Lower primates also a very wide range in size. Most of them, such as lorises, bushbabies, galagos, are small. Some lower primates, such as lemurs, are much larger and they are similar to monkeys in size. The lower primates have a wet nose pad, like that of a dog or cat. The wet nose pad allows them to smell better. Lower primates rely more on smell and they are color blind. In contrast, higher primates such as humans, monkeys and apes have color vision and a dry nose pad and we rely more on sight than the sense of smell. Some people do not know that lower primates and apes are not monkeys as they tend to think that all primates are monkeys, including the apes and lemurs. Scientists are more careful and they restrict the name monkey to the New World and Old World monkeys, and they do not use the term monkey to refer to apes like the gorilla and chimpanzee. Nor do they use the term monkey to refer to the lower primates like lemurs, lorises and the galagos.