The Buddha is an iconic image in world religion. Despite being a religion which at its heart encourages leaving behind all material concerns, iconography is widespread in Buddhist cultures. There are a variety of traditional poses in which statues of the Buddha are often found, each with its own meaning.
Buddhism takes its name from its founder, Siddhartha Gautama, who was born a prince in India approximately 500 years B.C.E. Though he lived an opulent life as a prince, he eventually came to renounce his material lifestyle. His teachings formed the basis for what would become Buddhism, and as such his image is revered. However, many "Buddha" statues are of other Bodhisattvas, a term which refers to anyone who has achieved enlightenment yet has chosen to remain on Earth and teach. While Siddhartha Gautama was the first, he was not the only one.
Buddhism started in India and spread to much of Asia, particularly East and Southeast Asia. A tolerant religion, as it spread from place to place it often took on regional characteristics. Thus, Buddhism in Japan is practised slightly differently than in India, China or the Southeast Asian peninsula. This in part explains why portrayals of the Buddha vary from region to region. Moreover, various local deities and figures were often incorporated into the imagery of Buddhism. Combined with the images of local Buddhas--that is, people who have achieved enlightenment--there is a wide variety in the Buddha statues found throughout Asia.
There is a wide variety of poses, but some are more common. The Buddha with his hands folded in his lap and his legs crossed, known as the lotus position, is the most common depiction. This is found almost everywhere Buddhism has taken root. Other frequently depicted postures include the Buddha with a raised hand or a hand touching the earth. Sometimes the Buddha will also be shown lying down in what is known as a reclining position. There is also a famous representation of the Buddha that has come to be known as the "laughing Buddha" because of his expression (and rather rotund belly).
Each of these different postures conveys a different meaning. The lotus position represents the process of meditation, an important aspect of Buddhism. If the hand is raised, it signifies a warding off of things that would cause fear. If the hand is touching the earth, it conveys the solidness of the ground in a metaphor for the stability of true Buddhist faith. The reclining Buddha is generally representing the final passage into nirvana, which is the state of nothingness to which some sects of Buddhists aspire. The laughing Buddha is based on a Chinese monk.
Regardless of pose, each Buddha statue serves essentially the same function--to inspire meditation and reflection upon the tenets of Buddhism. Different poses are designed to provide inspiration for different aspects of Buddhist teachings, to be sure, but they are all essentially meditative aids. Size is not really a key issue, though those statues that occupy a central place in temples do tend to be larger. However, there are often smaller representations of the Buddha dispersed across the temple grounds, to provide inspiration wherever a worshipper might choose to go.