Since InDesign's upper limit for a page size is 216 inches and 6 meters comes out to approximately 236 inches the job should be prepared smaller and the costumer will have to output it larger. I would suggest preparing it at half size so that it is a simple matter of outputting at 200%.
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Do you know who will be producing the banner? They will usually have specifications and possibly templates available regarding size, scaling and file formats amongst other things. It's also worth taking into account the likely viewing distance when deciding what final printed resolution images should be.
I did some banners almost the same size the other day and worked at 12.5%, with 300ppi images, but that was being produced in-house and I knew the viewing distance. I prefer not to design at full size for large format stuff. Composition-wise, I think that the amount of the viewer's field of vision occupied matters more than physical size. Optical Kerning, for instance, presumes they'll be holding something close, so it gets messed up when you work on large format stuff at full size.