Just wanted to point out that the accepted answer has a couple of limitations (which I discovered when I tried to use it)
It is thus not suitable (without adaptation) for use in a repeated-call setting (eg a ComponentResizedListener, or a custom/modified LayoutManager).
The listed code effectively assumes a starting size of 10 pt but refers to the current font size and is thus suitable for calling once (to set the size of the font when the label is created). It would work better in a multi-call environment if it did int newFontSize = (int) (widthRatio * 10); rather than int newFontSize = (int)(labelFont.getSize() * widthRatio);
Because it uses new Font(labelFont.getName(), Font.PLAIN, fontSizeToUse)) to generate the new font, there is no support for Bolding, Italic or Color etc from the original font in the updated font. It would be more flexible if it made use of labelFont.deriveFont instead.
The solution does not provide support for HTML label Text. (I know that was probably not ever an intended outcome of the answer code offered, but as I had an HTML-text JLabel on my JPanel I formally discovered the limitation. The FontMetrics.stringWidth() calculates the text length as inclusive of the width of the html tags - ie as simply more text)
I recommend looking at the answer to this SO question where trashgod's answer points to a number of different answers (including this one) to an almost identical question. On that page I will provide an additional answer that speeds up one of the other answers by a factor of 30-100.