The Heleriya's about one kilometer long. A five-kilometer long Colta would be more than sufficient to get the point across. Also, a five-kilometer-long Colta would have somewhere between 100 and 150 times the displacement and would require 100 to 150 times the amount of resources, putting it on the high end of the believable spectrum.
If you were to go for a 2.5 kilometer Colta, it would be perceptibly large enough to make any starship captain tremble. It would also have somewhere between 12 and 18 times the Heleriya's displacement, making it a more balanced foe.
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