You may find a smaller specimen, but they max. out around 5 inches. So as with most fish available in the hobby, you need to be aware that they'll grow some. It may not be pushing the limits now, but it's close and will be eventually in 40B. A lot also depends on feeding / wc regimen / and filtration. No 2 systems are alike in what they can accomodate. just my 2 bits.
I think I have the heaviest Bioload on this site. I do 10 gallon water changes every week and I have an oversized skimmer. My tank is a 65 and is well stocked up. Although 2 fish will be taken out shortly.
4 out of 3 people are bad at math.