jButton[i].setPreferredSize(new Dimension(42, 42));
There are 3 easy ways to make buttons larger.
- Use a big
Icon
for the button - Use a big
Font
for the button (preferably via a PLAF). - Set a large margin around the text and icon.
As to setting the preferred size. See Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing? (Yes.)
If you are still intent on going that route, take the advice of @trashgod and override the preferred size instead.