Max average size is about 17". There are some Lake Turkana fahakas that don't seem to grow beyond 3-4". And, a couple of members have fahakas that passed the 20" mark.
^^ What he said. My fahakas have always been more active and interactive in their own 120. Plus, they're messy feeders that love to tear their food apart. A bigger tank helps to keep water parameters at optimum for this species.