How do you use ZPL to change the font size to 9pt?
Onboard fonts are measured in dots, so you have to take into account the DPI of the printer you are targeting, typically 203 DPI, 300 DPI, or 600 DPI.
The ZPL Manual explains this and gives examples. See the third and fourth parameters of the ^A command.
Zebra also offers you a free WYSIWYG design tool called ZebraDesigner Free. You can create a label design and export the ZPL code. This is a great way to save some time and learn ZPL in the process.