First, don’t capitalize sun. (You’re tired of reading that? Well, I’m tired of needing to say it.)
Second, change was to were. (The least jargon-filled version of an explanation: The described scenario is extremely unlikely if not downright impossible, so you should use were instead of was. Compare I wish I was a millionaire to I wish I were a dragon.)
Capitalize both words in Milky Way.
If the sun were scaled won to the size of a white blood cell, the Milky Way would be the size of the continental United States.