Non-coding transcripts can be as small as 10-15 nucleotides. Once the RNA polymerase initiates, the DNA is melted and the transcription bubble forms. This region is about 10 nucleotides long.
But if you are asking the limit of mRNA length, then the answer is different. First of all, to get a protein, the ribosomes have to translate the transcript. To do that, they need some space to land on, and initiate the translation process. We know that ribosomes protect about ~30 nucleotides from ribosome footprinting experiments
Moreover, bacteria wouldn't need to utilize the genomic space and the ribosome machinery to produce small peptides (even if the peptides were useful), since the small peptides are already around as the degradation product of other proteins. Hence, once you add 3 times the shortest produced polyaminoacid length to the minimum length required for the ribosome landing, you get somewhat physical (or let's say physiological) limit of the transcript length.