Zumwalt already has strike-cruiser scale

How much larger would Zumwalt need to become to serve as a modern strike cruiser?

Very little; it already has strike-cruiser scale and strategic-length launchers, though too few ships were built to realise the concept at fleet level.

The answer treats Zumwalt as a strike cruiser in function despite its destroyer label. Its Mark 57 launchers can accept long weapons, while the original concept paired missile strike with heavy gunfire. The ship was intended to approach defended areas with air-defence escorts and deliver concentrated land attack. Its problem is not insufficient physical scale but the tiny class size and the programme changes that stripped away the original fleet concept.