It seems to be that the highest horizontal dimension you may chose for a timeline in Davinci resolve is 8192 pixels. If you will feed 15000 pixels wide DPX file to Resolve 8.2.2 on Windows it will accept it. The maximum horizontal size of the image that resolve will produce in render will also be 8192 pixels. On my PC with 1x Q4000 (GUI) and 2x GTX 5800 (3GB) render speed was about 6 fps (I do have fast storage)
It does not look you will be able to use resolve to grade the full size of your 15000×1080 footage.
May I recommend After Effects or Nuke (I think not sure but I think Nuke should be able to handle the size).
After Effects ships with Color Finesse a very powerful color grading plugin that will handle the frames of that size as well. That is being said, my PC has lots of RAM (48GB). You results may vary.
Dmitry Kitsov,
72mm Production