A lot of it depends on factors like how active vs sedate the individual is, and if the bird is "used to" a more modest size cage or not. In general, a healthy budgie is so active (even hyper) that although tiny, they really IMHO should have as large a flight cage as possible.
On the other hand, my Red Bellied parrot has been used to (and quite comfortable in) a 24x22" for a couple of decades! Even then, he doesn't use every inch of the cage. He's pretty sedate. He's free to come out a lot of the time, but even when his door is open, many times he chooses to sit inside on a favorite branch gnawing on a toy for some time. But back to your question about budgie cages, budgies are most definitely a species that need as much room as you can afford to give.