There are basically two ways to do it. One is with a script, and the other manually. The first is a bit more complex so I'll explain how to do so manually.
Go to Form Edit mode and select the fields you want to edit (either manually with the mouse, or via the Fields List panel). Right-click one of them and select Properties. Go to the Appearance tab and select the font size you want to use. It will be applied to all of them when you press Close.