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.
Is your component inside a panel with a layout manager? A layout manager will reset the size of your component based on various constraints. You can use the setPreferredSize() method instead of setSize() to tell most layout managers what size the field should be.
If what David said doesn't do what you want it to, check out the setColumns(int) method of JTextField; It should set how many characters you want to be able to fit in the field, which I believe will make it longer.