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.
In terms of tire size, your practical choices on new bikes are between (a) 700c, (b) what's called 27.5 inch (or French 650b), and (c) conventional 26 inch. These are ISO sizes 622, 584, and 559, respectively, where the ISO size is the actual diameter of the rim in mm (whereas most other size numbers are plucked from someone's posterior and have no relation to actual tire size).
The "29 inch" wheel is a 700c wheel is an ISO 622 wheel. (Oddly, the old "27 inch" wheel is 630 mm in diameter, slightly larger than the "29 inch" wheel.)
So, anyway, if you are looking at a "29 inch" bike that is a 700c -- the standard modern road bike.
In terms of "advantages", for a given sort of service a larger diameter tire will usually be made slightly narrower, to maintain roughly the same size "contact patch". This very slightly reduces wind resistance. Further, the larger tire rolls more smoothly over road irregularities, both providing a smoother ride and permitting slightly higher tire pressure (which, within reason, reduces rolling resistance). These differences between a 700c tire similarly designed 26 inch tire would be minimal, though -- tread design, tire width, and personal preference for tire pressure are much bigger factors.
In particular note that tire width is typically less on a road bike vs an off-road one. This is because the additional width is unneeded in terms of traction on loose surfaces and in terms of "flotation" at relatively low pressure over rough surfaces, and width costs efficiency in terms of weight, wind resistance, and rolling resistance.
There is also the point that a bike designed for the larger tire will usually have a somewhat larger frame, "tuned" to road vs off-road use, permitting a road posture that is more efficient in terms of both pedaling efficiency and wind resistance.
(Do note that if you are indeed interested in a "city" bike vs a road bike most of the considerations of efficiency go out the window.)