Let me add mine.
Before we say 'GO' or any other word, phrase, whatever is the shortest sentence, we have to define fully what sentence is.
My own definition of a sentence:
"GIVING A COMPLETE THOUGHT A SENTENCE IS A FULLY OR PARTIALLY EXPRESSED GROUP OF WORDS".
By using this definition, we can divide sentence into a fully expressed(all its words are explicit) and a partially expressed(some of its words are ommitted. They are either implied or undertood).
For example: 'John is happy' is a fully expressed sentence.
'Go' is a partially expressed sentence. The ommitted word implied is 'You'.
'In the room' an answer to 'Where is John?' is also a partially expressed sentence.
The words ommitted are understood.
Where is John? (He is) in the room.
The word 'yes', 'no' or 'ok' can be a sentence if the words ommited are understood.
We could say: "to determine whether a word is a sentence or not, depends on the context in which it is contained".