Changing default font size in text field in Adobe Acrobat Pro

These are two different things. You can change the default font size by editing a field and then right-clicking the field and selecting "Use current properties as new defaults". This will not affect any existing fields, though. For that you can use a script (like this one I've developed: Custom-made Adobe Scripts: Acrobat -- Mass Edit Fields Properties ), or by selecting the fields manually and then editing their properties manually.