JavaScript to resize a selected object in Illustrator!

Correcting myself: If you manually select a clipping path plus the placed/raster item it clips, it does appear in `selection` as a single GroupItem. (AI is smart enough to recognize you want them treated together. It’s when you select non-clipping groups that `selection` unhelpfully gives you all the individual paths instead.)

With that in mind, what you want is to set the clipping path’s width and height to your desired dimensions, then proportionally scale and reposition the image so that it completely fills that clipping path with any excess width/height cropped. The following code should handle that one particular use-case:

var doc = app.activeDocument;

var artboard = doc.artboards[0];
var rect = artboard.artboardRect;
var artboardWidth = rect[2] - rect[0];

var sel = doc.selection;


function calculateClippingPathHeight(width) {
  return width * 0.8; // use whatever calculation you want here
}

function recenterItemInBounds(item, bounds) {
  var p = item.position;
  p[0] = bounds[0] - (item.width - (bounds[2] - bounds[0])) / 2;
  p[1] = bounds[1] + (item.height - (bounds[1] - bounds[3])) / 2;
  item.position = p;
}

// assume a clipping path with a single placed/raster item behind it
if (sel.length === 1 && sel[0].typename === 'GroupItem' && 
    sel[0].pageItems.length === 2 && sel[0].pageItems[0].clipping) {
	
  var group = sel[0];
  var clip = group.pageItems[0];
  var image = group.pageItems[1];
  
  // set the desired width and height of the clipping path
  clip.width = artboardWidth;
  clip.height = calculateClippingPathHeight(clip.width);
  
  // calculate proportional scale factor for image so that it fills the clipping path 
  var scale = Math.max(clip.width / image.width, clip.height / image.height) * 100;
  image.resize(scale, scale);
  
  // recenter the image in the clipping path
  recenterItemInBounds(image, clip.geometricBounds);
  
  // recenter the group on the artboard
  recenterItemInBounds(group, artboard.artboardRect);
  
} else {
	alert("Selection is not a clipped image.");
}

 Developing a more general-purpose script which can process any number and combination of selected items, position items according to different rules, etc is left as an exercise to the reader.