In addition to general guidelines, heater size, in addition to tank size, needs to also take into account additional variables:
-> ambient (room) temperature. (colder rooms will typically require more heater energy to keep a tank at 76-78F~ give or take. A barely capable heater will be on a lot to maintain temperature in cooler rooms.)
-> whether the tank is covered. (Open tops loose more heat)
-> whether the tank is insulated (usually not the case, but I have Styrofoam surrounds on my tanks in my unheated basement).
-> A slightly larger wattage heater will work less to maintain the desired temperature and in larger tanks, two heaters split the work load even further.
Isn't it peculiar how something so seemingly simple can get more involved the deeper you dig?!