I agree that a small tank is tougher to keep stable than a big one. On the other hand, it is great training for a big tank. Things tend to get worse faster, like evaporation driving the salinity up, temperature fluctuating, nutrients causing algae blooms, and so on. If you are willing to pay the necessary daily attention that a little tank needs, you will learn a lot relatively quickly.
It also depends on what you want. You can have success if you start out with "easy" corals like mushrooms and zoanthids, plus tough crustaceans like hermits. You're very, very limited if you are thinking about fish, though. Maybe a few neon gobies, but that's about it.
Another vote for about 15 lbs.