Uranium just makes the radiation more effective and powerful. It depends on what warhead you're using, for range and effect of the explosion/radiation.
The Titan II ICBM in the 50s had a fifty kiloton warhead On it, I think. We used it for testing and the Titan project was later used for taking satellites high into orbit.
Behold, the bringer of [url=http://en.wikipedia.org/wiki/Tsar_Bomba]light[/url].
Thats with '60s tech too. Im sure if we wanted we could do better now days. Not to mention the far future.
Ive never really thought about the ships scale, I always chalked it up perspective