I have changed the size of a jQuery Datapicker using font-size, but my jQuery-UI icons (prev/next) are still their original size (although when I ohver onto them, the div size is really huge). How can I make the icons fit the new size of the datapicker?

The next and prev icons are background images displayed as part of a sprite:
You would need to get a bigger sprite set or create your own next/prev icons and override the background attribute of ui-icon.