I need to know if there is a method to enlarge the object by giving width and let height to increase proportionally.
Only that line of code would suffice.
I do not think there is, you have to calculate a proportion factor yourself and use any of the resizing methods such as transform() , resize(), or just setting width & height properties.
if I understand you right, you can try something like this:
var pI = app.activeDocument.pathItems; var newWidth = 150; var theFactor = newWidth/pI.width*100; pI.resize (theFactor, theFactor);