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The cooler star should be the larger... Your explanation is poor though and in a 5 mark question you might only get 1 mark for giving the right answer with a bad explanation.
Imo you should have been talking about apparent magnitude (not power) luminosity, temperature, diameter and cross sectional area of the stars and distance to observer (same for both in a distant binary obviously)
the question is a chance for you to show off what you understand about what affects the amount of light coming out of a star.
it's a real example you can look it up to confirm the accepted sizes of the stars in herc 95