the wattage of something like that would vary by nearly an order of magnitude depending on what you consider safe....
seems to me you could radiate at minimum 1000 watts safely, perhaps even twice that.
some examples:
300w Milwaukee Ribwound 1-1/8" x 8-1/2"
400w Milwaukee 26-136 Ribwound, 1-5/8" x 8-1/2"
150w Ribwound WW Resistor. 1-1/8"D x 4"L
500w Milwaukee Ribwound, 1-5/8" x 10-1/2"
420w Milwaukee 18-188 Ribwound Resistor, 1-1/8"D x 11-3/4"L
550w Ribwound WW 26-188, 1-1/2"D x 12-1/2"L