Follow the source train.
The image linked was sourced from an Urban Terror forum that made me feel dirty just visiting, then referenced a redditor, whose comments and posts I would rather not dig through thus ending my train of reference... I digress, it seemed their math was based on their knowledge that the white blood cell diameter was about 7 micrometers (the diameter of a lymphocyte (Wikipedia), which most associate with white blood cells) which changes the calculation quite drastically. 7 μm is about 0.000000004349 miles which changes your first result to 198,735,341,500,000.
That would make the galaxy 3253.57 miles and the average width of the United States is about 3,075 miles (wiki.answers). It all depends on where you get your numbers!