The word "short" can have several implied meanings. Basically, the word "short" is used to denote something which lasts for a very small interval of time or is of small duration, for example, a short movie, a short advertisement etc. In this case, the antonym of the word "short" is long as it indicates an event which lasts for a longer duration. People find it easy to sit through and withstand short movies or films. Long movies are bound to be monotonous and the viewer generally loses interest in them as time progresses.
Another implied meaning of the word short is a small or tiny stature. Here, tall will be the opposite of the word short. People with a tall stature or height find it easy to stand out of the crowd. Being tall is always an asset but a short height should not be used to undermine someone's credentials and abilities.