BGK can get 20-22" in captivity, I've seen it before..
Tank size is not an issue either, mine was in a 55g for 10+ years.. They aren't a 'space sensitive' fish like Pangasius or Pacu, so they aren't going to stress from lack of space - infact they probably feel more comfortable.. (notice they enjoy confined spaces, so that proves even more space is not an issue.)
The growth variable is MUCH more determined on diet and water quality.. the size of the glass box means nothing (effects water quality, though) .. tests have been done to show fish don't grow to the size of the home and unless they are a 'space sensitive' (ie; constantly swimming and have good visual skills) then it shouldn't matter. UNLESS, they use their electrosensory and the lack of space causes acute stress - but because of their natural behavior, I highly doubt they care as long as they have an appropriate sized hideaway..