Sorry, it's a bit confusing as originally the output window dimensions were the only way of controlling the exported size image/video size, but now there's the export options as well. Basically the output window dimensions are the base size when the export scale is set to 100%. So if you wanted to render at 4k you could have the output window dimensions set to 512 x 512 and then the export scale set to 800%
I might remove the output window dimensions since I think it's kind of confusing, as now it's really more of a ratio/crop area than anything, since the export dialog has more control over the final size