When converting and enable "preserve aspect ratio" DO should automatically calculate/show either y-pixels when typing x-pixels and otherwise.
One issue there is when batch converting, and you want all the images to fit inside the size you define (which is what the resize parameters really define, even when doing a single image).
Still, it might be nice to see what the result will be for the previewed image somewhere; maybe under where it has the input image dimensions and type now.
You're right, I completely forgot the batch-option 
!
But maybe we can save presets in future (like rename)?