I’d make it 18 deep x 17 wide and 17 high. I like to have room for a good grate and ash pan. And sometimes I get a split or two that are a bit longer. At one inch difference, One way or the other won’t really matter. Other than that,
IMO, I wouldn’t recommend making it longer/taller in any one direction. You need depth for splits and height so you can get airflow under your fire.