2" square tubing, 0.25" thick
How did I come up with that? By doing some simple Google searches that you could do, too. Don't forget in your calculations that you need to determine what width acrylic you will be using, whether there will be euro-bracing, how much substrate and live rock. Make sure your floor can withstand the total weight of tank, stand, sump (if in stand) and water. Have a professional weld the frame for you. At this much weight you can't afford mistakes.
Dave.M