Medium is the place to start. Small is too hard, too limiting.
Lots of reading, and then some more. Conscientious Marine Aquarium by Robert Fenner, New MArine Aquarium by Mike Paletta are good starters. Don't buy anything before these.
it is easier to start in the medium range because if you go to small then if your tank catches some desease or something it spreads so fast that there would be no time to stop it but if the tank is larger you have time to stop it
Start with at least 30 gallons. As recommended before, get The New Marine Aquarium by Mike Paletta. Also start with live rock, and a good protein skimmer.
Sam Reef
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90 Gallon FOWLR Marine System
20 Gallon Reef
10 Gallon Reef
20 Gallon Freshwater planted aquarium