How much space do I need? How much do I have?

So Im brand new to the hobby. I don't have a reef tank yet, I have been researching fish, corals, invertebrates, and such. I am a little confused when it comes to min tank requirements. For example. If I have a 300 gallon tank and one fish says it needs a minimum tank size of 180 gallons and another fish says min tank requirements of 120 gallons does that mean I can only have two fish and nothing else? Or does that mean you can only have one of that kind in that amount of water? Also, do i have to take into account invertebrates and corals and subtract their requirements and tank space from the over all available gallons? I don't want an overstocked tank but I also don't want a 300 gallon tank with one fish. Another thing I have been looking at is Powder Blue tangs. Of course because of this I have thought of building a tank that is 15 feet long 3 feet deep and 2 ft tall. While researching this fish I found that it needs a minimum tank size of 125 gallons. However, Most articles say that because of the difficulty with keeping this fish only people with "huge" tanks should try it.... My question is, what is considered a "huge" tank? is a 300, 400, 500 gallon tank considered huge? or is my plans for the tank I described just starting to boarder on huge? thanks all :)