Scientists have done experiments to study how the performance scales with size. Broadly speaking, the answer is 'the bigger the better'.
Why is this? The analogy of a camp fire is a useful comparison.
Imagine making a camp fire that is the size of a golf ball. It is difficult to keep it burning. The size of a tennis ball is a bit easier, but a fire the size of a soccer ball will produce a lot more heat and be harder to extinguish.
Similarly with a tokamak it is better to have a larger volume of plasma. Not only is the heat retained better because the volume rises faster than the surface area, but impurities are likely to have less chance of contaminating the plasma and the fuel particles can be kept in the plasma longer so that they have more time to meet and fuse together.
None of the machines built so far have been big enough to achieve better performance than breaking even (which would be called Q=1). In other words we are not yet producing more energy than we put in. JET is around about the size needed to reach 'break-even' and the next time it is operated with deuterium and tritium it should be able to break its own world record (Q=0.65) because the understanding of the performance of the plasma has improved since 1997 when the record was set.
ITER will be ten times larger than JET and it is expected that it will produce about 500 MW of power (at Q=10 or so). The only question is how easily the larger volume of plasma can be controlled. Don't worry - it only weighs as much as a few postage stamps so even though it is enormously hot it will not escape, but until it is in good control the machine would not be considered reliable enough to be a good power station.
Don't hold your breath. A machine as big as ITER will take another 10 years to be ready for operation. But starting construction is a step forward, at least!
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