I am doing a cross country road trip and wondering the largest size cargo carrier I can put on my CX-30. I am looking at the Thule Motion XT L (large size) but I haven't ever seen that on a CX-30 before. It's 77 inches long.
I have the Curt XL rack. It's the base rack with the extension. 63x47 approximately. You'll have to adjust the crossbars from the default position a little bit. Plus wrapping the bars with rubber spacers helps reduce pressure points.
I’ve heard one recommendation is to have the front sloped area in line with or behind the windshield to reduce buffeting. You could raise the tailgate and run a tape measure to see what would fit without interfering with the tailgate and aligning with the front. I have a Yakima rack on mine. It’s not large.