The Zebra SDK does not provide extensive label design support. It is designed to help with discovery, communication, configuration, and printing to Zebra printers. To my knowledge there is no real element of the SDK that will help you change font sizes dynamically.
Your approach is correct: defined a .LBL template file, and populate the data at print-time. Often, an end-user wil want to use a nearly identical label (in terms of font size, graphics, positioning) and will simply change the printed data. Dynamically changing those elements (font size, for example) doesn't seem to be a common need. That is just my impression.