Yes, Salisbury definitely doesn't seem like a 100,000+ town. For example Wikipedia gives Reading at around 175,000 and Salisbury definitely seems considerably smaller than Reading.
And while I don't know Newport very well, 300,000+ does indeed sound like a vast overestimation; if it were that big, it would be a pretty major regional centre. That figure isn't for Newport, Rhode Island?
(EDIT apparently not: Newport, RI is much smaller than I thought it was, and smaller than Newport, Wales; as I've heard of the place I imagined it would be a pretty big city).