fit_to_margin.jsx

(function (thisObj){
  main();
function main () {


   var sel = app.activeDocument.selection;
   if(sel.length < 1) {
       alert("no selection");
       return;
       }

  var numofcols = 1;
   var page = sel[0].parentPage;
   var margPrefs = page.marginPreferences;
   var columns = margPrefs.columnsPositions;
//~     alert( columns[1] + margPrefs.left  - columns[0]);

    var simpleDialog = app.dialogs.add({
      name: "Fit to Margins",
      canCancel: true
    });
    var list = [];
    for(var i = 1; i <= margPrefs.columnCount;i++){
      list.push(String(i) + " column");
    }

    var sdiag_column1 = simpleDialog.dialogColumns.add();
    var modeDropdown = sdiag_column1.dropdowns.add({
      stringList: list,
      selectedIndex: 0,
      minWidth: 75
    });

   if (simpleDialog.show() === true) {
    numofcols =  modeDropdown.selectedIndex + 1;
      simpleDialog.destroy();

  }
   var mwidth = columns[1] - columns[0] ;//  62.333 ;
   var w = (mwidth * numofcols) + (margPrefs.columnGutter * (numofcols - 1));

   fit_to_margins (sel,w);
}

   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);