When attempting to model ships from other universes in the BattleTech ruleset (who says my MicroMachine Star Destroyers can't fight a Blakist Task Force in the Nebula California? :) ), I use the old Heavy Metal Aero software and plug in various weights, which results in a new length being generated. Through trial and error, I can get the length in HMA to match the canon length of the ship being modeled, and then I use that weight as the basis for trying to fit all that ship's armament (be it turbolasers or Type IX Phaser Arrays) into the hull.
Here's a chart showing the WarShip progression:
Tons / Length
50,000 / 221 meters
100,000 / 284 meters
150,000 / 335 meters
200,000 / 381 meters
250,000 / 424 meters
300,000 / 463 meters
350,000 / 501 meters
400,000 / 537 meters
450,000 / 571 meters
500,000 / 605 meters
550,000 / 638 meters
600,000 / 670 meters
650,000 / 701 meters
700,000 / 731 meters
750,000 / 761 meters
800,000 / 791 meters
850,000 / 820 meters
900,000 / 849 meters
950,000 / 877 meters
1,000,000 / 905 meters
and so on, up to
2,500,000 / 1,652 meters (this is the cap for HMA)