I don't know about the details of why size matters, but in terms of producing energy: "When supplied with 300 MW of electrical power, ITER is expected to produce the equivalent of 500 MW of thermal power sustained for up to 1,000 seconds" [1]

That's thermal power, not electricity, so that's not enough yet to break even as a power plant, but it's a big step in the right direction.

[1] https://en.wikipedia.org/wiki/Fusion_energy_gain_factor