TSN is a collective term for Pseudoplatystoma genus
http://www.planetcatfish.com/common/genus.php?genus_id=116#778 whose the species have different max sizes.
There is disagreement between the scientists as to how many members are in that genus but definitely more than one. What we get in the pet fish trade 99%+ appear to be Pseudoplatystoma fasciatum and the tiny % is composed of tigrinum and reticulatum. We almost never get punctifer, neither corruscans.
The latter is arguably the biggest of the TSN species:
Spotted Sorubim
Pseudoplatystoma corruscans
FishBase max 166 cm; 100 kg.
Fishing-World-Records 180-250 cm, 86-150 kg.
Barred sorubim
Pseudoplatystoma.fasciatum
FishBase max 104 cm, 70 kg.
Fishing-World-Records 120-170 cm; 20-70 kg.
Tiger sorubim
Pseudoplatystoma tigrinum
FishBase max 130 cm, 17 kg. Matures at 99+ cm.
Fishing-World-Records 150 cm; 40 kg; 18 years.
But in captivity, chances are yours will not exceed 3'. Yet, even at 3', an 80" x 80" means 1-2 tail flaps in any direction and the fish must turn. That's awfully tight.
Many report their sub-adult TSNs are sedentary or rather sedentary. IMHO, this is, in at least 90% of cases, because of a too small tank. My TSNs and others in large accommodations love to swim around. They take rests, sure, but I'd not call them sedentary by any means.