indesign canvas can not be 6500mm in CC
can not create a canvas with 6500mm, it reverts back to 5486.4 mm
As an industry standard software, you would expect that there is no limitations to the canvas size.
Uwe Laubender commented
you could work around the issue and use a spread with several pages ( up to 10 ! ) where each of the pages are positioned individually on the spread using the Page Tool. That could be done best with NON-facing-pages documents.
( ACP )
Six and a half metres is a very high limit. From experience, file sizes can get huge at this scale. When I'm designing bigger than that I design at half-size and even quarter size. What are you trying to produce?