If you want big fish you'll need a big tank.
My largest fish is a 10inch long common pleco. My second largest fish is a common goldfish which is around 7-8inches long now.
Goldfish are awesome. I went fish shocking one time in the Potomac River (in Virginia) and we caught a 18" goldfish! It was huge! I guess someone had dumped their tank in the river and the fish just like the type of water.
But my biggest fish is a 5" silver dollarr. But I want to eventually have a 3' Arowana!