prob around 3ft if it has tsn, some hybrids grow even bigger than either parent due to hybrid vigour... will need a 2000+ gallon system in the end bud, massive investment to care for properly and not fair to buy it unless you are loaded and resposible to be honest!-)
could be a regular tsn anyway, they look spotty like that when small....
I'd agree with the ID, TSN (P. fasciatum) x Leiarius marmoratus. They are common. Mine and Chris the Aquaculture's are ~2.5'. I don't think they will exceed that unless kept for 10+ years. Then 3'+ may be possible. In proper care they reach ~1'-1.5' in the first year, 1.5'-2' in 2 years.
These are juvi tsn/redtail in this photo
The photo you put up op is tsn/marble I believe I would say the local fish store is right on and they arnt a common cross but you'd need a decent foot print I would say you can get away with a 8x4 foot print if you bought 1 cat
I certainly wouldn't buy both though