If you try to increase the paper size to very large dimensions, you may receive the message: "The page width/height you entered was invalid. It must be a positive, non-zero measurement, maximum for page and margins is about 9ft (2.75M), minimum about 0.5inch (1.3cm)."
There is no need to make your page this large. If, for example, you wanted a 4M poster you can scale it to 50% and have the page size set to 2M. Edit your file as required and print it at 200% and it will render everything perfectly, exactly as if you'd created it that size originally.
Therefore the size you make your poster really doesn't matter. You can create an A0 poster at A4 size and then just enlarge at print time. Or you can even give your printer a PDF file, for example, at A4 and tell him to print it A0 and it will come out fine.
This also works for bitmaps. The program retains the original high-resolution image at all times, and will use that when exporting or printing. So, unlike other bitmap programs, there is no loss in scaling down and scaling back up again.