We all know what he means, but he's not asking the right question. In he case of acrylic, it can literally be as large as you can handle. It comes down to cost, space, time, effort, and knowledge. First, if you've never worked with acrylic before, you'd better go buy some o practice with. Second, if you have worked with acrylic before, you have a good idea of your skill level. If you are planning to build an all acrylic tank you need to know the largest and thickest sheet of acrylic you are confortable working with and go from there, that will determine the maximum size of your aquarium. Not to burst your bubble, but in my opinion, building a massive aquarium out of acrylic as your first project is not a wise investment of your money. Build a small one and gain some knowledge from your inevitable failures and successes before trying to go extreme with acrylic. Use the glass thickness guide that is a sticky in this forum to determine the thickness required for your plan, build and learn.