Just to answer my own question, there doesn't seem to be a specific size as such, it seems what ever fits will do. I measured the ball joint and found the small diameter to be 13mm, the large one 29mm and the joint height to be about 20mm. The height might vary from front to back if the trackrod end is not level, in which case take the average value. Sellers on eBay list the dust covers as small, medium & large, and my values are covered by the small size. Semi-fluid grease is recommended by the Factory for the steering rack boot and since I have a 3kg tin I filled the covers with that before putting the clips on. Plenty of videos of how to install on YouTube.
