Minimal length is shorter than the optimized regulatory space. Shown here is the length distribution of divergent regions in S. cerevisiae that are conserved in other yeast species. The minimal length of 172 bp is defined by the lower limit of the length distribution (Table 1) and plotted in red; the coincidence of the first peak with a canonical promoter space of 140 plus two times of median length of 5'UTRs (blue vertical line) defines putative optimized regulatory space and is longer than the minimal distance. This optimized space is slightly shorter if it's calculated using another source of 5'UTR median length (grey vertical line)  but still longer than 172 bp.
Chen et al. BMC Genomics 2011 12:320 doi:10.1186/1471-2164-12-320