Is there a limit to how big before the effect falls off, or will we see twenty gigawatt net power output mega-tokamaks with rings five stories tall?

Fission seems to like to be big too but for different mostly logistical reasons. It makes all the nasty and hard aspects a lot easier if you do a lot of it on one site rather than a little on many sites, and it avoids having to ship nasty fissile material around too much.