First of all welcome to PFury
Fish don't grow to fit in their tank: their growth may become stunted, meaning that it will stay smaller, but it's a very bad condition for a fish to be in - could easily result in death over time.
So fish growing according to the space you give them is a myth.
For now, your tank should be enough. How long they can stay in your 33g depends on the tank's dimensions: if it's only 10" deep, you obviously can't keep a 7 or 8 inch piranha in there, let alone two.
Redbellies grow fast, reaching 5-7" in their first year, so they'll outgrow their current tank quite fast.
For life, I'd get at least a 40x18x18" tank, which provides enough room to swim around freely and to turn around (and freak out) properly. Reds can grow to at least 12" in length, so full-grown they're pretty big fish that need a tank to match.
If you get a somewhat larger tank, at least 48x18x18" in size, you could add one or two more Redbellies, which in general is a good thing to do: Reds are territorial fish that establish a pecking order. That means sooner or later one fish will become the alpha, the dominant fish. To make sure he's seen as the boss, he might start picking on the submissive fish, and when you only have two Reds, the submissive one will get the full beating (with severe injuries or even death as possible end-result). When you have 3 or more Reds, the agression is spread amongst more fish, which increases the chances of survival for the less dominant fish.
