Are tall bikes more efficient? I am considering getting a cruising bike to swap out riding with my existing, shorter mountain bike. I saw one I liked, for several reasons, in the store that is a 29" size. I am tall and could handle the tall bike, but the question is - is there a practical advantage with a taller bike? I live on FLAT terrain, in town. Thanks.
Different wheel sizes have different advantages/disadvantages.  Larger wheels tend to roll over obstacles better (more comfort), and be more efficient once up to speed.  Smaller wheels (generally lighter) are easier to spin up to speed, more maneuverable for highly technical terrain and often thought of as stronger (owing to shorter spoke lengths).

While it's true that many (almost all) cross country and trail bikes have gone to the 29" format, many other categories have not or have only moved slightly away (650b|27.5").  Downhill rigs are now largely 650b or still 26".  Dirt jump frames are still largely 26" format as well.  Due to the nature of these disciplines, the smaller wheel format is more appropriate.

Like anything else in cycling, equipment should be properly matched to use.