Howdy,
I may be in the minority here, but I believe that an aquarium should provide the best environment for the fish, and shouldn't be just a cage to house it in. The best environment is shown by nature, and I rule out drought as a tank scenario!

(although naturally occurring, it is tremendous stress on the ecosystem)
It is sad to see large fish in tanks that are too small for them to make even two strokes before hitting the wall. If it was a cat or a dog, people would have much more empathy for the animal.
Who would house a German Shepherd in a 6'x6' bathroom 24/7?! Those fish are
kings where they come from, and they are being kept like prisoners. They may be healthy and big, but they are still not kings.
Having said that, this is the
MonsterFishkeepers. A noble hobby and a difficult task. Go big guys.
If in doubt, go bigger! Not only our fish should be monstrous, but so should also be our tanks! Having a big fish in a tank where it cannot swim freely without bumping into glass is not cool, it's not monstrous, and it's certainly nothing to brag about. As a matter of fact it's a shame.
To answer the original question in general terms: I tend to upgrade my tanks when the fish grow to exceed 1/10-1/7 of the total tank length. That depends on the species, but is a good rule of thumb. If you cannot provide for the fish you want to keep, then you shouldn't get them. Just like you shouldn't get a German Shepherd if you can only offer it a 6'x6' bathroom to live in.
HarleyK