Would a 90% confidence interval be wider or narrower than a 95% confidence interval?

A 90% Confidence Interval would be narrower than a 95% Confidence Interval. This occurs because the as the precision of the confidence interval increases (ie CI width decreasing), the reliability of an interval containing the actual mean decreases (less of a range to possibly cover the mean). Look at 100% confidence; in order to ensure the mean is captured with 100% certainty, the interval must contain every possible value.