Apologies to those who have discovered it already but:
On the right hand end of the index line, the one immediately below the main banner heading, there is the letter A enclosed in up and down arrow heads (pretty inconspicuous). If you click that you can cycle through three different font sizes. Jolly useful to set a larger one if like me you are running a highish resolution on a smallish screen. Once selected it your choice stays set until you change it again.
One word of warning though. It affects the display of the text in the banner so if you are sensitive to that like Tom Bishop be prepared for it.