Going to disagree pretty wholeheartedly here.
1) there are a lot more factors to consider beyond simply marine/freshwater and aggression level
2)Doing research prior to getting fish (or any other pet) is hardly a bad thing.
Some things to consider when you start planning your aquarium - where will it be located - on a countertop or table, or on a designated tank stand? How close are you to a sink to facilitate water changes (carrying enough buckets to do a proper water change on a 55 or 90gal tank clear across the house gets old quick). Typically, the answer is "as big as you can fit/afford". A larger tank is generally more stable from a water parameter and temperature perspective and is less susceptible to sudden changes. However, everything is more $$$ with a larger tank - more decor, more gravel, larger filter, heater, lights, etc. So, it will partly be driven by budget. If you have space a 29gal or a 30gal long are decent starter sizes in the area of, not too expensive, but large enough to give you a lot of options for stock and decor.