The shortest English sentence is 'it is'. It has a noun and a verb, which is what every sentence must have. 'It' is the noun and 'is' is the verb.
Well, actually, the shortest English sentence would have to be I am or I go or I do, any of which is one letter shorter. Grammatically speaking, Go as a command is also a complete sentence, as is Be. If you include unqualified interjections, then the hands-down winner would have to be O!