It depends on why you want them to fill the screen with the barcode.
If you are trying to get a representation of the barcode that can be rescanned from the phone screen then I would seriously recommend that your system should read the scanned code and recreate it rather than use a scanned image. This way you'll get an optimum black and white image rather than a photo that may not have been taken under the best lighting conditions.
If, on the other hand, you just want to make it easier for you system to understand the barcode then most apps use a visual guide to help the user understand how to scan the code.
A lot of them opt for either of two methods:
A marked crop area
Or a scan line
In some cases both are used