That's going to depend a lot on what you determine to be enough energy and speed for deer sized animals. My first thought would be 6.5 PRC Bullets and brass are available for it, so you will have options, lots of options. This will likely be important when choosing to go with a non-standard length. My load developed for a 24.3" Tikka is projected to push a 156 EOL at 2575 fps in a 16". That would get you to 800 with about 1700 fps and 1000 ft/lbs of energy. I would personally call that a cutoff point (if not slightly sooner). If you wanted to get to 1000, I think you're going to need to get a bunch more case capacity, and you're gonna lose a ton of efficiency then. Are you looking to run suppressed? If so, you will likely be bumping up against barrel length restrictions for your can.