what kind of barcode you are using to generating. i am using this barcode creating sdk to create all kinds of barcodes. for code 128 barcode, it is able to change the size of the barcodes including the height and width. it is important to set the right ratio. check the following sample codes and tutorials.
http://www.rasteredge.com/how-to/vb-net-imaging/barcode-generating/
http://www.rasteredge.com/how-to/vb-net-imaging/barcode-generating-code128/