Trollfish 5 is the biggest I've seen, although it's not that much bigger than a size 4 tuna. Size 5 tuna is going to be enormous when I finally find one.

the lowly pike is impressive too.
https://steamcommunity.com/sharedfiles/filedetails/?id=2930140238

I haven't even tried the new fishing, and it is a T4 Pike, which is actually like twice the size of me. I was standing in shallow water, and it jumped over me. So I grabbed it! I was shocked at the size of it once I put it on the wall. I seriously said, "I'm going to need a bigger wall!" 😁

I think I've seen 5's but thats about it

T5 trollfish had to run to the other end of my longship so i could keep it. the stamina requirement is absurd like 10s/s absurd.

found a t5 puffer fish on a rock.

To be fair, it was a shore pick-up... t5 grouper:

https://steamcommunity.com/sharedfiles/filedetails/?id=2905280181