Most adults will need a full-size (4/4) violin, and a full-size bow to go with it. Younger teenagers may need a 3/4 size, and smaller sizes are available for children.
In general, the larger the instrument the better the sound, but if the instrument is too big you may find it uncomfortable and difficult to play. This is particularly important for younger players, who can strain themselves if they have to stretch their arms and fingers too far.
As a guide, you should be able to comfortably hold the scroll in the palm of your left hand while the holding the instrument on your shoulder.