Could you please specify where you want to use this exactly?
If you are calling the object for a mashup, then you can useqlik.app.getObject method. You can use this method to insert the object into an HTML element. After that you can resize the element as you want using the element attributes.
If you are using older versions(<=June 2018) of Qlik Sense then you will first have to embed the object into an HTML element, then define the height and width. After that you can export the image using following module.