The size of a reactor depends on the capacity of the plant, the technology selected to convert thermal energy into electrical energy, and the technology in the core and fuel designs. Let's consider a commercial nuclear plant.
In siting a plant, a utility would balance the capacity of a plant with the demands in the system and the design of the electrical grid. In the beginning of the nuclear power industry, the plants were relatively small, of the order of 50 MWe to several hundred MWe. By the mid 1970s, the main designs were about 1000 to 1100 MWe, although some were as large as 1250 MWe. Some plants had built in margin, which has allowed increases in capacity of 5 to 20%. For some modern plants, we've seen capacities between 600 MWe (AP-600) to 1100 MWe (AP-1000), up to 1600 MWe (EPR/US-EPR) for Gen-III+, as well as a new drive to modular units that have capacities in 50 MWe to 300 MWe, which offer smaller plant footprints, and may be sited closer to populated areas.
With the capacity determined, then one looks at the thermodynamic cycle, e.g., Rankine (LWR or CANDU), Brayton (gas turbine), or combined-cycle (Brayton/Rankine). No one has built a combined-cycle NPP.
The core technology depends on the type of fuel and neutron spectrum (moderation), which determines power density, temperature and pressure. Then one has to select the materials for the fuel and core structures that will provide for an economic service, and one must consider the radiation effects and service life of the fuel and core structural systems. Corrosion, and to some extent, erosion, must be minimized, and balanced against temperature, which determines thermodynamic efficiency.
Looking at transport issues, one should look at existing plants, e.g., 17x17 fuel in 193 or 205 assemblies per core, versus a small modular reactor, like NuScales design. It has been pointed out that large nuclear reactors are typically built near water ways (many are coastal) whereby the large components arrive by barge or special ship. Smaller modular reactors offer the possibility of rail, assuming the rail lines and bridges are capable of handling the capacity.
One can find descriptions of plants, including large reactors, small reactors and advanced (non-LWR) concepts, here -
https://www.nrc.gov/reactors/new-reactors.html
Usually, the pressure vessel containing the core, is transported empty. The fuel is installed after the plant is constructed. Some small modular designs offer the potential to ship the integrated primary system (PV and core) loaded with fuel. This would be challenging since one has to ensure integrity of the control elements.
NuScale design - https://www.energy-northwest.com/ourenergyprojects/smr/Documents/NUSCALE%20UPDATE_Modern%20Power%20Systems_Sep2016.pdf
Since the OP asked about 'large' reactors, then one could consider the AP-1000, APWR, OP-1400, EPR and/or VVER-1200, and perhaps the ABWR/ESBWR. One can search on any of the these designations and find some basic information.
