I think it just means that the whole reactor itself IS a module, can be removed, put on a truck, repaired, upgraded, and replaced as needed. Here is a Wikipedia link that has a picture of one. https://en.wikipedia.org/wiki/NuScale_Power
Since convection depends on absolute height, reactors relying on convection have a certain minimum height. Roughly speaking. So if you scale a reactor down, at some point it becomes very skinny and that might lead to problems. Anyway, 60 megawatts electricity is not too big in my opinion.
