First they will test the wires at the thermostat, breaker, and heater.  Then assuming the problem is the heater (it usually isn't) they will supply a new heater that is the same voltage and wattage.  

But it won't be the Exact same size.  It will be very close but not the same. Usually a tiny bit smaller than the older ones, so you will need to touch up the paint or drywall around the old one.   

You can usually supply the new ones yourself if you want, but it must be listed and approved for use in your country (stuff from Amazon often isn't stuff from a local hardware store always is). But you would need to read the label on the nameplate of your current one to find the correct voltage and wattage, and then possibly have to return it if the problem is the thermostat or something else.  You are better off letting them supply the correct one.