I remember spending hours trying to catch that big guy in that pond.
How do you catch him and was there an optimal way to catch that lunker?
What is his maximum size?
How big could the normal fish get to be?
According to Zelda Wiki:
The Hylian Loach is a large, rare fish that appears in both Ocarina of Time and Twilight Princess. The loach looks like a cross between the catfish and the pike, with an eel-like body and ornate swirl patterns down the length of its body. They generally weigh in at about 29-36 pounds and can only be found at the Fishing Holes in its respective game. Due to its rarity, the Hylian Loach is usually revered to be a legendary fish.
In Ocarina of Time, the loach appears purely by random at the fishing pond in Lake Hylia. The best way to find it is by equipping the Zora Tunic and the Iron Boots, and then entering the water and scanning the sand floor and the area around the lily pads. If it can't be found the first time, Link can just exit the area, enter again, and repeat the process. They can be caught with either the regular lure or the Sinking Lure, although the loach will only bite onto the hook as it goes up to the surface.
The only way to catch the loach is to get the sinking lure near the log. You will see an eel-like fish, then cast your rod with the Sinking Lure and wait. It is very hard to catch him, so be quick. He is the biggest fish in the pond. It took me several times to catch this legendary fish, so good luck.
It took me 3 hours to get that fish! Every 4 times you enter the pond put on your Iron Boots and Zora Tunic. Scan the pond floor around the logs and lily pads. Once you find him go walk around the stream and waterfall to find the sinking lure. Go over to where you found him and fish for him like you would any other fish. Once you get him the owner will pay you 500 rupees and then throw the fish back in the pond.