There's no easy way to achieve this, and generally any solution is going to be a little hackey and violate some normally recommended solitons...
You Could...
Create a second temporary button with three characters and use it's preferred size as the preferred size of button to b displayed...
You Could...
Get the font metrics from the button, calculate the height and width of the text you would normally otherwise display, add in the insets and margins and set this value as the preferred size of the button...
FontMetrics fm = button.getFontMetrics(button.getFont());
int width = fm.stringWidth("MMM");
int height = fm.getHeight();
Insets insets = button.getInsets();
Insets margins = button.getMargin();
width += insets.left + insets.right + margins.left + margins.right;
height += insets.top + insets.bottom + margins.top + margins.bottom;
button.setPreferrdSize(new Dimension(width, height));
Personally, if I had to do this, I'd go with the first option :P