What is the shortest mRNA the ribosome can read to produce a peptide?
Reading the paper linked by @canadianer and its references was pretty illuminating. It's a straightforward paper except for its conclusions, and the most convincing part is the evidence of actual translation. There's some evidence that transcription start sites are used as a regulatory mechanism by displacing other things that might otherwise bind. An unknown fraction, but most(90%+), very short bioinformatically detected ORFs are not translated.

Some, however, are. See this table for the experimentally verified translated micropeptides. The shortest is a nearly unbelievable 6 peptides long.