It's not a system limitation. You can make text areas as big as you want.
That said, if I'm understanding it right, you are not using rulers to mark out your text area? That's what I do if I want something a certain size. Mark it out with rulers and use snap to when drawing my text area.
edit: Interesting, I see what you mean in the area type options. I just drew out my text area which was about 800mm wide, and then went to the options and it said 352.78mm and reset the size to that when I clicked OK. I don't know if there is a proper way around that, but you can always draw them out bigger by hand, as long as you don't need to use the text area options box.