[gmueckl]

The real issue is surface to volune ratio of the contained plasma. This needs to be as small as possible for the containment to be effective (reduced energy losses through the surface area). So you want your reactors to be as big as possible.

[pfdietz]

From an engineering point of view, though, you want the reactor to be as small as possible. That's because output is limited by the power/area that the first wall can withstand. So, the volumetric power density will be inversely proportional to the linear dimensions of the reactor.