Nuclear reactor cores can be very small, like the size of a snow globe (see Clementine), especially if they use highly-concentrated fissile material like plutonium or highly-enriched uranium. But if it's making more than a few hundred watts of power, it will typically be radiation-shielded by about 8 feet of high density concrete on all sides. This shielding will generally weigh around 200 tonnes or more. So if you're seeing reactors on or the size of trucks, keep that in mind.

We ran a truck-mounted in Idaho once before, the ML-1. It required lots of superhigh purity demineralized water (to avoid neutron activation) and earth to be built up around it during operation to avoid significant radiation doses 500 ft. away