An office printer does not go beyond A4. Choose the grid and the screen tells you how big the poster will be once glued.
What it does not do: Crop marks are not printed: the glue strip is the guide. And the printer adds its own unprintable margin, which this tool knows nothing about: allow for it in the strip. The pages are redrawn: clickable links, bookmarks, form fields and accessibility tags do not survive. And what falls outside the frame stays in the file without being shown, as with cropping: an editor can bring it back.
Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.