I have an answer (which you may or may not like) and a question for you.
First, the answer. I would suggest you may want to consider working with Adobe Illustrator. Its max dimension is 5779.6mm, which gives you a little more room to work with as you design your banners. It's not a lot, but it's something ...
Now for the question: Do you really need to design a document that large? If you're sending the job out the job to a vendor for production, sign/large format producers have workflows which let them upscale jobs automatically far past the limitations of either InDesign or Illustrator. You may want to check with the folks who will produce your banners to see what their requirements are for producing your banners.
It's not unusual for large format producers to work in 10:1 (or for folks in the inch-foot world, 12:1) ratios to produce your work. Actually many, if not most, vendors prefer it. Just realize that if you're going to be working at 1/10th scale to produce your original art, you need to work at 10x pixel image resolution to get the final results you're looking for (e.g. – to meet a vendor's resolution specification of, say, 75ppi for rendering pixel images at final size, you're going to have to supply that art at an effective resolution of 750ppi when creating original art which will be blown up at a 10:1 ratio for final output).
At any rate, your vendor(s) will be happy to help you get the best possible results for your design efforts, because it's in their best interest to keep you as a happy repeat customer. Ask them what they need, and they'll be happy to share what they need from you for best results.
Typically work at 10% size - so your document size would be 600mm and then the printers output at the correct size which gives you the 6000mm. This is a very normal practice - just in your file name state that it's <Filename_10percent.pdf>
Of course you still need your images (raster) to be at least about 300ppi in your document set at 600mm - so on increase in size they are 30ppi which is normal enough for large size banners.
Working at 10% is usual practice - but always double check with your print provider.