Barcode

I have been trying to change the barcode size, but am having difficulty as it is hard to judge how big the barcode should be, i have looked at other magazines and seen the average size of one but it is hard to judge when on Photoshop. I had no trouble importing the image as a new layer and using the transform tool to adjuct the size (whilst holding shift to keep is the same aspect) however, i am unsure on what size it should be.

This isn't a massive problem as the barcodes found on college mags are traditionally bigger or even non-existant. However, for my final i will have to have done more research and gone into more effort to get the barcode the right size.