This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You may need another layout. You could try GridBag, with insets round the JTextFields, or Google for MigLayout.
You may be able to set sizes on the JTextFields; they probably have several methods like setXXXSize.
I adjusted and compiled the code you posted, and it works. Is it because you left the default layout for JPanel instead of giving it a GridLayout? Maybe I have to fiddle some more with that and see what works and what not...