With the name and size or model number of the water heater, you can get the proper anode rod.
Simply google, as an example: ;Suburban 6 gallon anode rod; and you will get lots of hits.
PS: I have a COLD water heater ... if the water is already hot, no need to heat it.
Not trying to be a wise turkey, just the term "hot water heater" doesn't make sense to me although many people use that terminology.