Hutch,
In this photo, I am standing next to the chains (don't ask me on which mast, because I don't remember; the photo was taken back in 2006) on HMS Victory.

By comparing the size of the deadeyes to the size of my noggin, I'd guestimate them to have a diameter of maybe, oh … say 12 inches or so. At scale 1:60 that would be as near as makes no difference to 5 mm.
OBVIOUSLY a tea clipper in 1860 did not necessarily use the exact same size of deadeyes as HMS Victory does today, so just the above might with very good reason still be too imprecise for your needs. If you wish to be more exact, you would either need to find out somehow what size deadeyes were used on the real Ariel back in the day, or at least find out if there were standard sizes of deadeyes at the time. The latter is, I think, not unlikely, given that such objects were made by blockmakers, who probably worked to some sort of at least tradition or common practice in the trade. Lloyd's insurance regulations might also have specified minimum sizes to be used for different types of rigs or sizes of ships, and thereby created de facto standards. Likewise, Royal Navy rules and standards may have influenced merchant practices as well.
However, if you don't want to go to lengths of research on the subject, I would suggest that you decide on what seems a realistic size, expressed in inches (as that would surely have been the measurement used), multiply those inches by 25.4 to get the corresponding full size in millimeters, divide by 60 and then round off to the nearest size in whole millimeters available. That way, you're not likely to be too far off track.
Mattias
Edited By Banjoman on 05/02/2018 10:34:44