The issue of size is determined by a heat loss calculation. It is basically a measurement of all your heat loss surfaces ( walls ceilings windows doors) mutltiplied by known values for material/insulative value. At the end you get a BTU load for a typical maximum heat diff between outside and what you want inside. If a contractor does’nt do the arithmetic, ask him to, and if he does’nt know what you are talking about and rule of thumbs the job, fine a brainier operator.
Changing from steam to water is more efficient because of the efficiency losses of phase change and higher temps of steam. There are also more critical error that can be made unless you have a quality moomche.
The very best system if you can find someone to do it right is continuous circ. In this system, the pump always runs, the radiator temps are kept from tepid to warm, never hot. This makes the radiant effect of the radiator much higher, (comfort) doesnt stir up dust, reduces burn risk, and actually is most efficient except for the electrical load of the pump. The pump also lasts longer running at lower temp and continuously rather than start/stop, and the run of the boiler is controlled primarily by the thermostat, which is the most accurate part of the system. This is great for very large houses with high volume systems.
Squeak: there were several ways to do steam, some two pipe, and the science of how these work is a nearly lost art. Big, old, large pipe systems that worked by gravity only: hot water rises, cold water falls, all in big pipe with asbestos insulation. They worked without pumps, and some gas systems worked on milivolt contol that the pilot generated itself. In steam, this could be a electric less system. There were coal feeders that worked on thermal expasion/ contration of a bimetal sensor, and chains that also worked “by themselves” The laws of physics is a beautiful thing.
In a steam to water convert, some old radiators on steam don’t have the requisite tappings.
Lastly, good double pane windows, attic and walls insulated to the max, (and a tyvek house wrap if you are residing) are one time expenses that forever reduce your energy needs and have no moving parts (except windows)
As you might sense, this was my bread and butter before I moved to Jerusalem, where I can have tomatoes on the vine in January, but almost no heating business. L’toeles harabim, you can reach me for advice if the Mod lets.