You need to consider how many amps and what voltage you're trying to dump and then scale your resistance wire (can we just say "nichrome" from here on out?) so that it has the proper resistance for the task, using Ohm's law. Ohms = Volts/Amps.
It would probably be wisest to dismantle the toaster and construct a mount for the nichrome wire inside that would be more amenable to airflow to cool it off. Nichrome should handle up to about 2000 degrees F but as it heats up its resistance increases and things can get a bit squirrely from there.
Googling for "nichrome wire" will supply a lot more detail than I can offer.