Step 1: Open your PDF and select the Tools tab.

Step 2: Click on Prepare Form

Step 3: This will take you back to the PDF where you can select each fillable field.  Right-click and select Properties.

Step 4: In the window that opens, select the Appearance tab, and under Text change the Font Size.  You can manually select or you choose Auto it will make the text fit inside the box.  Click Close once you are done and save your document as normal.