so frequency and magnitude
of the current and the area of any loop in the circuit all contribute.
What loop ? I think you are talking about an inductor.
Wires carrying current generate magnetic fields which is why they are kept separate from signal wires but I never heard of any one
talking about a power switching device like a mosfet generating magnetic fields. I'm sure there is some magnetic field but it isn't anything to be worried about and certainly nothing to be talking about. Power devices are separated from other devices on pcbs because the traces carrying their current could affect small signal traces and also because there is some heat involved but location of power switching devices like mosfets is not chosen because of any concern about magnetic fields. All of those concerns are reserved for inductors, solenoids, and motors. If you have evidence to the contrary I would be interested in seeing it because this is the first time I have heard this issue brought up in 30 yrs of working with electronics.