To get the text to always fit the size of the form field, you can change the font size to auto within Adobe Acrobat.
In Adobe Acrobat, select Prepare Form.
Right-click the form field and select its properties. The Text Field Properties dialog opens.
Select the Appearance tab. The Text section has a Font Size drop-down list, a Text Color swatch, and a Font drop-down list.
Open the Font Size list and select Auto. The list starts with Auto, followed by fixed sizes such as 6, 8, 9, 10, 12, 14, and 18.
Select Close to save the change.