To adjust height, click on the drop-down menu for "Bar Height" and select "Custom" – the number entered is in millimeters and corresponds to the height of the longest bar. If you enter a number that is too small (below either our or the barcode symbology's minimum size standard, depending on the symbologies), the height of the barcode won't change and the number will appear in red.
In general, you should leave the height at its default option unless there's some specific reason to change it.
For more information on changing the height of UPC/EAN barcodes, see this specific article under FAQs.