OSX now permits pages of vast size.
I have reached 100 million inch square (2,540 km square) and not hit the ceiling.
So custom paper size is not the limit.
What will be a limit however will be if your file contains bitmap images. A bit map of 840 x 1000mm will be 100s of Mbs at a printable resolution and there is a ceiling limit to text of 1000 pts/352mm (not actual cap height).
If you have mostly vector art the file size may be kept down but Pages 5 or 6 is very inefficiently written and may grind to a halt. I have been able to send a well tanked up Mac Pro into a Spinning Beachball of Death by having too many objects in a file, it probably will do the same with a combination of very large bitmaps, text and other large objects as it tries to render it on screen.
Further, Pages is not a professional DTP app and does not produce the correct PDF format required by Professional Printers.
I would recommend Affinity Designer instead as being the professional solution probably closest to your budget. It is most like Adobe Illustrator so not quite a DTP solution but perfect for a poster of any size.
I have just tested a poster of your size in AD and it is certainly smooth and efficient in operation and more reliable and functional than the current Pages.
Peter