While I agree that bigger is better, I believe that a 29 gal
tank is the smallest size I would recommend. This is because may people do not have the space to accomodate a 4 foot tank. The smaller tank only means that it will require more constant attention. As long as your dedicated enough to do top offs every 1 or 2 days, and can limit feeding, you will be ok with a smaller tank.
I started a reef tank after keeping FW
planted tanks for years. I still can't believe, about a year later, how much easier a reef tank is for me than a FW
planted tank. The weekly maintenance is much less, water changes are less frequent, and tank cleaning is easier(except for coraline algae). I know a lot of people say that SW
is difficult, but IMHO
if you do your research, are on top of your tank parameters and have lots of patience, you will be a fine reefkeeper.