Welcome to R2R Adriana! I think the tank size depends on the livestock you're wanting. Tangs should be in a bigger tank, whereas clowns and other smaller fish do great in smaller tanks. I wouldn't recommend the standard 55 gallon, just because there isn't very much width to play with. Pretty much ANY other tank size works out better, just my opinion though. Start deciding now what equipment and salt you'd like to use. Salt is a neverending expense, and depending on the size tank you decide on, could run over 100 dollars a month.