How to change the font size for strings in a JTable?

table.setFont(new Font("Serif", Font.BOLD, 20));

Try implementing your own Custom Renderer, then you'll be able to treat each string as a JLabel and use setFont(...) accordingly.

One option is to set UIManager hints before GUI initialization, e.g.:

FontUIResource font = new FontUIResource("Verdana", Font.PLAIN, 24);
UIManager.put("Table.font", font);
UIManager.put("Table.foreground", Color.RED);