What is the shortest complete sentence in English?

There are two main candidates for the shortest sentence in the English language. 

1. Go!
2. I am.
3. Okay.

Go! has (arguably) the strongest case. It has an active verb (t0 go), an implied subject (second person singular or plural) and a clear meaning. 

I am is only meaningfully a sentence when used as an answer to a question: Who is going to the Moon this morning? I am.

Okay - is an even weaker candidate, lacking an active verb and in some instances an implied subject. It is, however, sometimes used as a self contained response e.g. an implied agreement to complete an action or series of actions: How about you cook lunch and I'll hoover the bedroom?" he said. She nodded, 'Okay.

Who decides the rules of English?

Is there an English language rulebook?