My honest suggestion is read a lot, and then some more. 50 dollars spent on books at an early stage will save you hundreds of dollars further down the road. Marine aquaria are not especially tricky, but if you start with a bad plan, and do NOT trust your lfs to be of much help as the vast majority are not very good, your chances of expensive failure are very high.
I strongly recommend Mike Palettas New MArine Aquarium and Reef Secrets by Fossa and Nilsson. A lot of people recommend the Conscientious MArine Aquarist by Robert Fenner, but while all aquarists should read this book, it's getting old on setup. There are others.
Average sizes for marine aquaria are bigger than for fresh, I would guess a 55 is the average, but plenty of people have 75's and bigger.
I recently started a new tank, and described my experiences in a sticky at the top of this forum. Good luck