not to mention the concensus of experienced fishkeepers, and some basic logic
OP- i started with a 29 gallon biocube.. as far as your fist tank goes such an all-in-one is definitely good enough
the 20-30 gallon range is about as small as i would suggest you go for a first tank (not sure of your experience with FW...)
and it'll hold a decent ammount of fish and the lighting will let you get into the introductory species of corals... shrooms, leather, toadstools, etc should you decide to go into that
the only problem could occur if you intend to jump right into ]V[arine ]V[onsters
but if you have the money i'd recomend that over the smaller tank
things like lionfish, triggers, puffers, etc are all really hardy and are definitely ]V[onster!
