I would suggest you put together a kit such as the Folger Tech i3-2020. And then use it to print a few parts of your choosing. You will run into some problems and you will learn a huge amount working through the issues. After you get comfortable with your printer, it might make sense to start with a clean slate and design a printer with a large envelope. But really... you need some working experience on a functional printer first. Think of it this way: Why aren't there dozens of large envelope printers on the market? Why don't all of the 3D-Printer enthusiasts have large printers? The reason is the problems become much bigger as you increase the size of the print envelope. At least get some experience before you dive into the deep end of the pool!