Personally, I placed my sensors in the thermostat housing. It is where Kia put one so I followed suit.
I'll try and post size of pulley and picture of it.
Yes, but the purpose of these sensors is to determine the engine temperature, so it makes sense they should be right at the thermostat housing. They are sensing how hot the engine is, not how hot the radiator is. They OEM sensors weren't intended to control a radiator fan. The OEM radiator fan is controlled by the clutch, which is heated by its proximity to the radiator.
I am not sure if it makes more sense for the radiator fan to be controlled by the radiator temperature, rather than the temperature at the thermostat housing. If you're going down the road, the temperature of the water coming OUT of the radiator will be a lot cooler than the water going IN, so no fan is needed. If you're sitting still, however, the temperature coming OUT of the radiator is going to be much hotter, so you need the fan. In a way this seems like it makes more sense, to put the sensor on the cool side of the radiator. I'm not sure it matters a whole lot though, it's way easier to just connect to the OEM sensor, which is how i have it connected currently.
I just think there might be cases where my fan kicks in while the vehicle is moving down the road and doesn't need the fan.