Would a barium swallow test detect a medium to large sized esophageal ulcer? what about a narrowing/stricture?

Yes: Barium swallow or esophagram should detect medium or large size ulcers, narrowing/stricture. Upper endoscopy is however more sensitive.

It could. : A barium swallow would likely detect these things. Endoscopy would be more accurate with the added benefit of being able to biopsy what may be found. Often if symptoms are suggestive and barium swallow is negative we’d do the endoscopy anyway.