How tight should a triathlon/TT jersey fit?

Very tight—it should feel like a second skin with no wrinkles or loose fabric when in the riding position. You should be able to breathe normally, but the jersey should not slide or shift on your body. If you can pinch more than 1-2cm of fabric anywhere, it is too loose for optimal aerodynamics. The fit should be tested in your TT position on the bike, not standing in front of a mirror.