When gesturing, I believe it's best to keep things in some kind of proportion.
So when you're indicating a 10-gallon aquarium, your hands are out to about here. Then if you refer to a 20-gallon one, your hands need to go out farther. If you keep them where they were, that's still a 10-gallon tank, I'm sorry.
If it's a 50-gallon one, you can still manage it, probably, unless you're John McCain, but obviously it's a stretch. Work at it. If you're talking about a 50-gallon aquarium and you're only gesturing a 38-gallon one, that's no good. Because I don't think they come in 38-gallon models.