It can be small, big, placed on the moon or in a submarine—anything. Even a net positive energy balance is not required, for example, if the reactor functions effectively as a type of battery.
However, it must be an economically rational choice to achieve a given goal, which is not related to nuclear fusion reactor research itself, when compared to other options.
Furthermore, it must be realized and put to use.
I would appreciate it if you could comment on some possible use cases!