would a 30l biorb tank be big enough for the gold fish?
Not for long as it grows. Goldfish have a reputation as the ultimate short lived disposable pet, but they're actually pretty long lived (longer than a dog - 20-30 years isn't unusual of and 40+ has happened) and get big. Common breeds from good breeding can get well over a foot long, fancy goldfish (fat bodied ones) can get the size of a grapefruit, plus the fins. They're not difficult fish to care for, the only major concern is housing. 20 gallons (~75l) for a fancy goldfish is pretty minimal, and a non-fancy breed won't even be able to turn around in that full grown.
Just to add to the problem, biorbs (at least all the models I've seen) have that central pillar from the filter, which means the effective maximum fish size is smaller than it would be in a rectangular tank with the same volume.
Edit: Somebody else mentioned stunting: The limiting factor isn't tank size, but water quality. Big fish in small tanks overwhelm their filtration and water quality is poor, which causes stunting. By analogy, think if you lived locked in your bathroom with the toilet broken. Things would go south fast.
Using automated systems to rapidly replace water, people have done some pretty impressive things (academically anyway, as a pet keeper they're terrible things). TFH magazine had a writeup last year of 20 oscars being farm grown to almost full size in only 100 gallons - an automated system did the equivalent of a full water change every couple hours to maintain water quality.
Like I said, though, that sort of accomplishment is purely academic. Fish don't need space just for water quality - no amount of water changes or filtration makes a tank big enough for a fish that will ultimately get too big to move around in it.