Most thermostats use 24vac, but some could control 120vac or 240vac. Then, without knowing if your system has multiple zone control capabilities, you may not be able to add another thermostat without changing the controls at the boiler.
Exactly what does the thermostat turn on/off? A zone valve, a pump, or other?
In its simplist form, a thermostat is just like a temperature controlled on/off switch. There's power on one lead, when the thermostat determines heat is required, it closes and applies that power signal on the other lead - just like a wall switch to turn on a light. It depends on what that switch is trying to turn on.
Depending on the voltage on the wires, and the distance of the run, would determine the size and type of wire required. So, without knowing more about the system, it isn't safe to say. Often this is something like 18g bell wire, but it could require larger or smaller, or some other type depending on what the system design.