fit_to_margin.jsx

(function marginfitting(thisObj){
   
   var sel = app.activeDocument.selection;
   if(sel.length < 1) {
       alert("no selection");
       return;
       }
   var page = sel[0].parentPage;
   var margPrefs = page.marginPreferences;
   var columns = margPrefs.columnsPositions;
//~     alert( columns[1] + margPrefs.left  - columns[0]);
   var mwidth = columns[1] - columns[0] ;//  62.333 ;
   fit_to_margins (sel,mwidth);
   
   function fit_to_margins(sel,newwidth){
    for(var i =0;i < sel.length;i++){
            var curbnds = sel[i].geometricBounds;
            
            sel[i].geometricBounds = [curbnds[0],curbnds[1],curbnds[2], curbnds[1] + newwidth];
        }
    
    }

    })(this);