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That's 1.8 meters, or almost 6 feet in diameter.
Are you importing the tower stl into Fusion? That may be where it's going catty-wumpus. it would be better to mock up a dummy tower (just a few cylinders for the tower and stairs) that is the correct size, then build your ring. Meshmixer will accurately report the xyz dimensions of an stl. You can use those dimensions to mock up the placeholder tower. Besides, simple cylinders will render way faster.