Sounds like you want to be able to scale-up (or zoom-in) your
selection to the preview area.
You can do this with a cropper extension from Andy Stanberry. His
extension's main cause is to allow you to have multiple preview areas
on a page, but it also implements the zoom-in capability, so I am
using it with just one preview area just to get the zoom capability.
In fact you don't even have to set the minWidth and minHeight
properties.
http://www.negativemargins.com/javascript-ui-cropper-with-multiple-previews/